rm path from pubspec.yaml, it isn't valid

* Path results in pub not being able to find the onesignal_flutter package.
* Fixes the following error
   - pub get failed (66; Because onesignal_example depends on onesignal_flutter from path which doesn't exist (could not find package onesignal_flutter at "^3.1.0"), version solving failed.)
* Tested with Flutter 2.2.3

Author: jkasten2

example/pubspec.yaml

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