Fix the Google.Maps.WebTilesReceiver I/F definition in ApiDefinition.cs

As reported in Xamarin forum,
http://forums.xamarin.com/discussion/9020/google-maps-ios-components-v1-5-0-contains-bug-at-async-interface-of-tilelayer#latest
Google.Maps.WebTilesReceiver I/F contains bug in it's definition.

I changed it and it becomes working well.