Fix broken formatting in underscore.coffee docs

This is a straightforward fix to the broken formatting found on the official CoffeeScript site. The [top search result for 'Underscore CofeeScript'](http://bit.ly/google-search-underscore-coffeescript) is this page.  Having a well formatted project page makes the project that much more approachable to potential new adopters. No source files were changed. See screenshots below for comparison. The syntax highlighting, alignment, and responsive formatting have been fixed.

## Before
![broken](https://f.cloud.github.com/assets/1577682/1051297/88363f9e-10c8-11e3-868f-5d2a28f0ad7d.jpg)

## After

![fixed](https://f.cloud.github.com/assets/1577682/1051299/927659bc-10c8-11e3-9415-bd4a9daf4392.jpg)