Fixed a typo where "exist" should be "exists"

Exists in lines 222 and 248 matches the singular nouns it is being used with. "Exist" had a mismatch where it expected plural nouns.

![image](https://cloud.githubusercontent.com/assets/7017045/5586194/f64cf9e6-9079-11e4-97e6-d28226e1456f.png)