fixed block quote's css

The css of the 'blockquote' is not good, like this

![origin_blockquote](https://f.cloud.github.com/assets/3000006/2435905/d283ef6e-add1-11e3-94ed-5eae0c8b6afb.png)

after modified

![blockquote](https://f.cloud.github.com/assets/3000006/2435886/40b7f120-add1-11e3-8282-000cfb878b59.png)