fix typo in update measure error dialog

Story: https://www.pivotaltracker.com/story/show/77314672

Old: You have attempted to update a measure with a file that represents a **differnt** measure. Please update the correct measure or upload the file as a new measure.

New: 
![image](https://cloud.githubusercontent.com/assets/2136286/4139609/7c913516-339c-11e4-8951-517712c7d120.png)