[SPARK-15883][MLLIB][DOCS] Fix broken links in mllib documents

## What changes were proposed in this pull request?

This issue fixes all broken links on Spark 2.0 preview MLLib documents. Also, this contains some editorial change.

**Fix broken links**
  * mllib-data-types.md
  * mllib-decision-tree.md
  * mllib-ensembles.md
  * mllib-feature-extraction.md
  * mllib-pmml-model-export.md
  * mllib-statistics.md

**Fix malformed section header and scala coding style**
  * mllib-linear-methods.md

**Replace indirect forward links with direct one**
  * ml-classification-regression.md

## How was this patch tested?

Manual tests (with `cd docs; jekyll build`.)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #13608 from dongjoon-hyun/SPARK-15883.

1 files changed, 3 insertions, 3 deletions

diff --git a/docs/mllib-decision-tree.md b/docs/mllib-decision-tree.md
index 9af48357b3..11f5de1fc9 100644
--- a/docs/mllib-decision-tree.md
+++ b/docs/mllib-decision-tree.md
@@ -136,7 +136,7 @@ When tuning these parameters, be careful to validate on held-out test data to av
 
 * **`maxDepth`**: Maximum depth of a tree.  Deeper trees are more expressive (potentially allowing higher accuracy), but they are also more costly to train and are more likely to overfit.
 
-* **`minInstancesPerNode`**: For a node to be split further, each of its children must receive at least this number of training instances.  This is commonly used with [RandomForest](api/scala/index.html#org.apache.spark.mllib.tree.RandomForest) since those are often trained deeper than individual trees.
+* **`minInstancesPerNode`**: For a node to be split further, each of its children must receive at least this number of training instances.  This is commonly used with [RandomForest](api/scala/index.html#org.apache.spark.mllib.tree.RandomForest$) since those are often trained deeper than individual trees.
 
 * **`minInfoGain`**: For a node to be split further, the split must improve at least this much (in terms of information gain).
 
@@ -152,13 +152,13 @@ These parameters may be tuned.  Be careful to validate on held-out test data whe
   * The default value is conservatively chosen to be 256 MB to allow the decision algorithm to work in most scenarios.  Increasing `maxMemoryInMB` can lead to faster training (if the memory is available) by allowing fewer passes over the data.  However, there may be decreasing returns as `maxMemoryInMB` grows since the amount of communication on each iteration can be proportional to `maxMemoryInMB`.
   * *Implementation details*: For faster processing, the decision tree algorithm collects statistics about groups of nodes to split (rather than 1 node at a time).  The number of nodes which can be handled in one group is determined by the memory requirements (which vary per features).  The `maxMemoryInMB` parameter specifies the memory limit in terms of megabytes which each worker can use for these statistics.
 
-* **`subsamplingRate`**: Fraction of the training data used for learning the decision tree.  This parameter is most relevant for training ensembles of trees (using [`RandomForest`](api/scala/index.html#org.apache.spark.mllib.tree.RandomForest) and [`GradientBoostedTrees`](api/scala/index.html#org.apache.spark.mllib.tree.GradientBoostedTrees)), where it can be useful to subsample the original data.  For training a single decision tree, this parameter is less useful since the number of training instances is generally not the main constraint.
+* **`subsamplingRate`**: Fraction of the training data used for learning the decision tree.  This parameter is most relevant for training ensembles of trees (using [`RandomForest`](api/scala/index.html#org.apache.spark.mllib.tree.RandomForest$) and [`GradientBoostedTrees`](api/scala/index.html#org.apache.spark.mllib.tree.GradientBoostedTrees)), where it can be useful to subsample the original data.  For training a single decision tree, this parameter is less useful since the number of training instances is generally not the main constraint.
 
 * **`impurity`**: Impurity measure (discussed above) used to choose between candidate splits.  This measure must match the `algo` parameter.
 
 ### Caching and checkpointing
 
-MLlib 1.2 adds several features for scaling up to larger (deeper) trees and tree ensembles.  When `maxDepth` is set to be large, it can be useful to turn on node ID caching and checkpointing.  These parameters are also useful for [RandomForest](api/scala/index.html#org.apache.spark.mllib.tree.RandomForest) when `numTrees` is set to be large.
+MLlib 1.2 adds several features for scaling up to larger (deeper) trees and tree ensembles.  When `maxDepth` is set to be large, it can be useful to turn on node ID caching and checkpointing.  These parameters are also useful for [RandomForest](api/scala/index.html#org.apache.spark.mllib.tree.RandomForest$) when `numTrees` is set to be large.
 
 * **`useNodeIdCache`**: If this is set to true, the algorithm will avoid passing the current model (tree or trees) to executors on each iteration.
   * This can be useful with deep trees (speeding up computation on workers) and for large Random Forests (reducing communication on each iteration).