[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-ensembles.md b/docs/mllib-ensembles.md
index 2416b6fa0a..5543262a89 100644
--- a/docs/mllib-ensembles.md
+++ b/docs/mllib-ensembles.md
@@ -9,7 +9,7 @@ displayTitle: Ensembles - spark.mllib
 
 An [ensemble method](http://en.wikipedia.org/wiki/Ensemble_learning)
 is a learning algorithm which creates a model composed of a set of other base models.
-`spark.mllib` supports two major ensemble algorithms: [`GradientBoostedTrees`](api/scala/index.html#org.apache.spark.mllib.tree.GradientBoostedTrees) and [`RandomForest`](api/scala/index.html#org.apache.spark.mllib.tree.RandomForest).
+`spark.mllib` supports two major ensemble algorithms: [`GradientBoostedTrees`](api/scala/index.html#org.apache.spark.mllib.tree.GradientBoostedTrees) and [`RandomForest`](api/scala/index.html#org.apache.spark.mllib.tree.RandomForest$).
 Both use [decision trees](mllib-decision-tree.html) as their base models.
 
 ## Gradient-Boosted Trees vs. Random Forests
@@ -96,7 +96,7 @@ The test error is calculated to measure the algorithm accuracy.
 <div class="codetabs">
 
 <div data-lang="scala" markdown="1">
-Refer to the [`RandomForest` Scala docs](api/scala/index.html#org.apache.spark.mllib.tree.RandomForest) and [`RandomForestModel` Scala docs](api/scala/index.html#org.apache.spark.mllib.tree.model.RandomForestModel) for details on the API.
+Refer to the [`RandomForest` Scala docs](api/scala/index.html#org.apache.spark.mllib.tree.RandomForest$) and [`RandomForestModel` Scala docs](api/scala/index.html#org.apache.spark.mllib.tree.model.RandomForestModel) for details on the API.
 
 {% include_example scala/org/apache/spark/examples/mllib/RandomForestClassificationExample.scala %}
 </div>
@@ -127,7 +127,7 @@ The Mean Squared Error (MSE) is computed at the end to evaluate
 <div class="codetabs">
 
 <div data-lang="scala" markdown="1">
-Refer to the [`RandomForest` Scala docs](api/scala/index.html#org.apache.spark.mllib.tree.RandomForest) and [`RandomForestModel` Scala docs](api/scala/index.html#org.apache.spark.mllib.tree.model.RandomForestModel) for details on the API.
+Refer to the [`RandomForest` Scala docs](api/scala/index.html#org.apache.spark.mllib.tree.RandomForest$) and [`RandomForestModel` Scala docs](api/scala/index.html#org.apache.spark.mllib.tree.model.RandomForestModel) for details on the API.
 
 {% include_example scala/org/apache/spark/examples/mllib/RandomForestRegressionExample.scala %}
 </div>