Add newline to highlight code example in document

This is the same kind of fix as #3961.

* before
![2013-12-19 4 31 16](https://f.cloud.github.com/assets/290782/1776776/5c68619e-681b-11e3-82c4-8cc3f4c75018.png)

* after
![2013-12-19 4 33 03](https://f.cloud.github.com/assets/290782/1776775/5c680622-681b-11e3-9b9e-79878be5fd69.png)