Fix AlertIOS Docs

Summary:
The curly braces seems to be redundant.
Closes https://github.com/facebook/react-native/pull/811
Github Author: xcatliu <xcatliu@gmail.com>

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

Libraries/Utilities/AlertIOS.js

@@ -37,7 +37,7 @@ var DEFAULT_BUTTON = {
  *     {text: 'Foo', onPress: () => console.log('Foo Pressed!')},
  *     {text: 'Bar', onPress: () => console.log('Bar Pressed!')},
  *   ]
- * )}
+ * )
  * ```
  */