typo

_Original PR made on wrong branch (facebook/react/#1981)_

"Instance" needed to be pluralized. 

Made the edit in two places (line 478 & 484):

> Finds all _**instance**_ of components

to

> Finds all _**instances**_ of components

![image](https://cloud.githubusercontent.com/assets/2935356/3789840/50cd72d8-1ac8-11e4-9156-d2e21fbfcb42.png)