Fix table of contents width

This line causes the table of contents to bleed into the content area under Bootstrap's medium breakpoint (>= 992px and < 1200px).

![screen-shot-2014-06-14-at-11 07 39-pm](https://cloud.githubusercontent.com/assets/625922/3280107/ff0e7e60-f443-11e3-8841-30f44f49f6e0.png)