Update link to MainActivity.kt file (facebook#4533)

* Update link to MainActivity.kt file

Wrong url was used linking to MainApplication.kt file instead.
0.76-0.78 versions affected

* Update website/versioned_docs/version-0.78/_integration-with-existing-apps-kotlin.md

update the link to point to the 0.78

Co-authored-by: Bartosz Kaszubowski <[email protected]>

---------

Co-authored-by: Bartosz Kaszubowski <[email protected]>

Author: 1nspir3d

website/versioned_docs/version-0.78/_integration-with-existing-apps-kotlin.md

@@ -424,7 +424,7 @@ class MyReactActivity : ReactActivity() {
 </TabItem>
 </Tabs>
 
-As usual, here the [MainActivity.kt Community template file as reference](https://github.com/react-native-community/template/blob/0.77-stable/template/android/app/src/main/java/com/helloworld/MainApplication.kt)
+As usual, here the [MainActivity.kt Community template file as reference](https://github.com/react-native-community/template/blob/0.78-stable/template/android/app/src/main/java/com/helloworld/MainActivity.kt)
 
 Whenever you create a new Activity, you need to add it to your `AndroidManifest.xml` file. You also need set the theme of `MyReactActivity` to `Theme.AppCompat.Light.NoActionBar` (or to any non-ActionBar theme) as otherwise your application will render an ActionBar on top of your React Native screen: