Make trailing colon on class manipulation clear

I got stuck on class manipulation because I thought that the trailing colon on the documentation was just a typo and therefore didn't include it in my code. I think it should be more clear on the doc:

Before:
![image](https://f.cloud.github.com/assets/992008/1356340/b95ae5ce-3775-11e3-94e4-1e967cf688b0.png)

After:
![image](https://f.cloud.github.com/assets/992008/1356334/a2fcd67a-3775-11e3-9e45-c7d5ef7061b4.png)