Display the code editor as preformatted text

When going to the http://www.rust-lang.org/ with NoScript blocking all foreign assets, here's what my page looked like (since jsdelivr.net was blocked):
![screen shot 2014-12-12 at 23 43 26](https://cloud.githubusercontent.com/assets/632942/5422638/3b57ead2-825b-11e4-88aa-be8b026cea7f.png)

This PR is a one-line CSS fix (again without jsdelivr.net):
![screen shot 2014-12-12 at 23 53 58](https://cloud.githubusercontent.com/assets/632942/5422639/3f1e8dec-825b-11e4-9416-1e18dd3ed4b4.png)