Fix indentation in mock_channel

* Fix indentation in mock_channel.dart

Author: tanaynigam

test/mock_channel.dart

@@ -68,8 +68,8 @@ class OneSignalMockChannelController {
         this.state.externalId = null;
         return {"success" : true};
       case "OneSignal#setLanguage":
-      this.state.language = (call.arguments as Map<dynamic, dynamic>)['language'] as String?;;
-      return {"success" : true};
+        this.state.language = (call.arguments as Map<dynamic, dynamic>)['language'] as String?;;
+        return {"success" : true};
     }
   }
 }