Changed iOS to Xamarin.iOS in README

MapDemo/README.md

@@ -2,7 +2,7 @@ Map Demo
 =================
 
 This sample demonstrates how to use the MapKit framework to display a map
-view with a custom annotation.  This relates to the [iOS
+view with a custom annotation.  This relates to the [Xamarin.iOS
 Maps](https://docs.microsoft.com/xamarin/ios/user-interface/controls/ios-maps)
 guide.