GCM for iOs - Fixed issue on DeviceToken.md

@drool said: Documentation can be found here: https://components.xamarin.com/gettingstarted/googleiosgcm but I failed when tried to set DeviceToken as a value for key Constants.RegisterAPNSOption. Has anyone succeeded?

The document has error.

var options = new NSDictionary();
change to
var options = new NSMutableDictionary();

SendUpStream

public void SendUpstreamMessage()
		{
			var msg = new NSMutableDictionary();
			msg.SetValueForKey(new NSString("1234"), new NSString("userId"));
			msg.SetValueForKey(new NSString("hello world"), new NSString("msg"));

			var to = GCM_SENDER_ID + "@gcm.googleapis.com";

			Service.SharedInstance.SendMessage(msg, to, (_messageId++).ToString());
		}