fix css class name

fix small typo for bootstrap class
![bad_style](https://cloud.githubusercontent.com/assets/1040571/4751586/d8387be8-5a9f-11e4-941a-0594a45060c1.jpg)
![fixed](https://cloud.githubusercontent.com/assets/1040571/4751585/d838740e-5a9f-11e4-94a8-c540aa5ab69d.jpg)