Update README: MainActivity -> MainApplication
Update README: MainActivity -> MainApplication

Tested on a fresh install with react native 0.35

MainActivity doesn't have a method called getPackages for override, so that didn't make sense.
zmxv/react-native-sound,hayanmind/react-native-sound,zmxv/react-native-sound,zmxv/react-native-sound,zmxv/react-native-sound,hayanmind/react-native-sound,hayanmind/react-native-sound,gvenk/react-native-sound,gvenk/react-native-sound,gvenk/react-native-sound,gvenk/react-native-sound,hayanmind/react-native-sound