[SPARK-4931][Yarn][Docs] Fix the format of running-on-yarn.md

Currently, the format about log4j in running-on-yarn.md is a bit messy.

![running-on-yarn](https://cloud.githubusercontent.com/assets/1000778/5535248/204c4b64-8ab4-11e4-83c3-b4722ea0ad9d.png)

Author: zsxwing <zsxwing@gmail.com>

Closes #3774 from zsxwing/SPARK-4931 and squashes the following commits:

4a5f853 [zsxwing] Fix the format of running-on-yarn.md