Remove duplicate word

Summary:
Closes https://github.com/facebook/react-native/pull/768
Github Author: Tyler McGinnis <tylermcginnis33@gmail.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/StyleSheet/StyleSheet.js

@@ -47,7 +47,7 @@ var StyleSheetValidation = require('StyleSheetValidation');
  * Code quality:
  *
  *  - By moving styles away from the render function, you're making the code
- *  code easier to understand.
+ *  easier to understand.
  *  - Naming the styles is a good way to add meaning to the low level components
  *  in the render function.
  *