Fix filename in Android installation guide

Author: kadikraman

README.md

@@ -164,7 +164,7 @@ steps instead.
 
 #### Android
 
-1. Open up `android/app/src/main/java/[...]/MainActivity.java`
+1. Open up `android/app/src/main/java/[...]/MainApplication.java`
 
 * Add `import com.reactlibrary.RNAppAuthPackage;` to the imports at the top of the file
 * Add `new RNAppAuthPackage()` to the list returned by the `getPackages()` method