[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, 1 insertions, 1 deletions

diff --git a/docs/mllib-feature-extraction.md b/docs/mllib-feature-extraction.md
index 4c027c84ec..67c033e9e4 100644
--- a/docs/mllib-feature-extraction.md
+++ b/docs/mllib-feature-extraction.md
@@ -333,7 +333,7 @@ Details you can read at [dimensionality reduction](mllib-dimensionality-reductio
 
 The following code demonstrates how to compute principal components on a `Vector`
 and use them to project the vectors into a low-dimensional space while keeping associated labels
-for calculation a [Linear Regression]((mllib-linear-methods.html))
+for calculation a [Linear Regression](mllib-linear-methods.html)
 
 <div class="codetabs">
 <div data-lang="scala" markdown="1">