[SPARK-16485][DOC][ML] Fixed several inline formatting in ml features doc

## What changes were proposed in this pull request?

Fixed several inline formatting in ml features doc.

Before:

<img width="475" alt="screen shot 2016-07-14 at 12 24 57 pm" src="https://cloud.githubusercontent.com/assets/717363/16827974/1e1b6e04-49be-11e6-8aa9-4a0cb6cd3b4e.png">

After:

<img width="404" alt="screen shot 2016-07-14 at 12 25 48 pm" src="https://cloud.githubusercontent.com/assets/717363/16827976/2576510a-49be-11e6-96dd-92a1fa464d36.png">

## How was this patch tested?

Genetate the docs locally by `SKIP_API=1 jekyll build` and view it in the browser.

Author: Shuai Lin <linshuai2012@gmail.com>

Closes #14194 from lins05/fix-docs-formatting.

docs/ml-features.md

@@ -216,7 +216,7 @@ for more details on the API.
 
 [RegexTokenizer](api/scala/index.html#org.apache.spark.ml.feature.RegexTokenizer) allows more
  advanced tokenization based on regular expression (regex) matching.
- By default, the parameter "pattern" (regex, default: \\s+) is used as delimiters to split the input text.
+ By default, the parameter "pattern" (regex, default: `"\\s+"`) is used as delimiters to split the input text.
  Alternatively, users can set parameter "gaps" to false indicating the regex "pattern" denotes
  "tokens" rather than splitting gaps, and find all matching occurrences as the tokenization result.
 
@@ -815,7 +815,7 @@ The rescaled value for a feature E is calculated as,
 `\begin{equation}
   Rescaled(e_i) = \frac{e_i - E_{min}}{E_{max} - E_{min}} * (max - min) + min
 \end{equation}`
-For the case `E_{max} == E_{min}`, `Rescaled(e_i) = 0.5 * (max + min)`
+For the case `$E_{max} == E_{min}$`, `$Rescaled(e_i) = 0.5 * (max + min)$`
 
 Note that since zero values will probably be transformed to non-zero values, output of the transformer will be `DenseVector` even for sparse input.