Update example/pubspec.yaml to use local onesignal_flutter plugin

- This was a problem if someone doesn't set the local path in `pubspec.yaml`
- Running and testing the example app won't actually run the new changed code in the SDK

Author: nan-li

example/pubspec.yaml

@@ -16,7 +16,8 @@ dev_dependencies:
   flutter_test:
     sdk: flutter
 
-  onesignal_flutter: ^3.1.0
+  onesignal_flutter:
+    path: ../
 
 # For information on the generic Dart part of this file, see the
 # following page: https://www.dartlang.org/tools/pub/pubspec