Improve markup in CONTRIBUTING.md

Before:

![2013-06-11 1 03 49](https://f.cloud.github.com/assets/290782/632601/6cf88dce-d1e7-11e2-961b-3cb8dafbe186.png)


After:

![2013-06-11 1 04 07](https://f.cloud.github.com/assets/290782/632603/88fbc180-d1e7-11e2-9b12-508b7e4903f4.png)