update doc for android MainApplication.java

Summary: Closes https://github.com/facebook/react-native/pull/8886

Differential Revision: D3586269

fbshipit-source-id: d6dc0a3618f3c1c4516ce428dd525b3f8aa09d3c

docs/NativeModulesAndroid.md

@@ -117,7 +117,7 @@ class AnExampleReactPackage implements ReactPackage {
   }
 ```
 
-The package needs to be provided in the `getPackages` method of the `MainActivity.java` file. This file exists under the android folder in your react-native application directory. The path to this file is: `android/app/src/main/java/com/your-app-name/MainActivity.java`.
+The package needs to be provided in the `getPackages` method of the `MainApplication.java` file. This file exists under the android folder in your react-native application directory. The path to this file is: `android/app/src/main/java/com/your-app-name/MainApplication.java`.
 
 ```java
 protected List<ReactPackage> getPackages() {