change link in add-to-app module readme (#464)

add_to_app/flutter_module/README.md

@@ -7,4 +7,5 @@ An example Flutter module used in the Flutter add-to-app samples.
 For more information, see the Add-to-App guide at
 [flutter.dev](https://flutter.dev):
 
-https://github.com/flutter/flutter/wiki/Add-Flutter-to-existing-apps
+For instructions integrating Flutter modules to your existing applications,
+see the [add-to-app documentation](https://flutter.dev/docs/development/add-to-app).

add_to_app/flutter_module_using_plugin/README.md

@@ -8,4 +8,5 @@ in the Flutter add-to-app samples.
 For more information, see the Add-to-App guide at
 [flutter.dev](https://flutter.dev):
 
-https://github.com/flutter/flutter/wiki/Add-Flutter-to-existing-apps
+For instructions integrating Flutter modules to your existing applications,
+see the [add-to-app documentation](https://flutter.dev/docs/development/add-to-app).