[SPARK-18198][DOC][STREAMING] Highlight code snippets

## What changes were proposed in this pull request?

This patch uses `{% highlight lang %}...{% endhighlight %}` to highlight code snippets in the `Structured Streaming Kafka010 integration doc` and the `Spark Streaming Kafka010 integration doc`.

This patch consists of two commits:
- the first commit fixes only the leading spaces -- this is large
- the second commit adds the highlight instructions -- this is much simpler and easier to review

## How was this patch tested?

SKIP_API=1 jekyll build

## Screenshots

**Before**

![snip20161101_3](https://cloud.githubusercontent.com/assets/15843379/19894258/47746524-a087-11e6-9a2a-7bff2d428d44.png)

**After**

![snip20161101_1](https://cloud.githubusercontent.com/assets/15843379/19894324/8bebcd1e-a087-11e6-835b-88c4d2979cfa.png)

Author: Liwei Lin <lwlin7@gmail.com>

Closes #15715 from lw-lin/doc-highlight-code-snippet.