[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, 5 insertions, 5 deletions

diff --git a/docs/mllib-linear-methods.md b/docs/mllib-linear-methods.md
index 63665c49bc..17d781ac23 100644
--- a/docs/mllib-linear-methods.md
+++ b/docs/mllib-linear-methods.md
@@ -185,10 +185,10 @@ algorithm for 200 iterations.
 import org.apache.spark.mllib.optimization.L1Updater
 
 val svmAlg = new SVMWithSGD()
-svmAlg.optimizer.
-  setNumIterations(200).
-  setRegParam(0.1).
-  setUpdater(new L1Updater)
+svmAlg.optimizer
+  .setNumIterations(200)
+  .setRegParam(0.1)
+  .setUpdater(new L1Updater)
 val modelL1 = svmAlg.run(training)
 {% endhighlight %}
 
@@ -395,7 +395,7 @@ section of the Spark
 quick-start guide. Be sure to also include *spark-mllib* to your build file as
 a dependency.
 
-###Streaming linear regression
+### Streaming linear regression
 
 When data arrive in a streaming fashion, it is useful to fit regression models online,
 updating the parameters of the model as new data arrives. `spark.mllib` currently supports