Merge pull request #551 from jungchris/patch-1

Minor update README.md - 'Extensibility' Section for iOS code (callback)

README.md

@@ -133,7 +133,7 @@ It is certainly possible to create a great app using React Native without writin
 - (void)processString:(NSString *)input callback:(RCTResponseSenderBlock)callback
 {
   RCT_EXPORT(); // available as NativeModules.MyCustomModule.processString
-  callback(@[[input stringByReplacingOccurrencesOfString:@"Goodbye" withString:@"Hello"];]]);
+  callback(@[[input stringByReplacingOccurrencesOfString:@"Goodbye" withString:@"Hello"]]);
 }
 @end
 ```