Fix type-o in MobileVRInterface sample code in docs: xr -> use_xr

Author: dsnopek

modules/mobile_vr/doc_classes/MobileVRInterface.xml

@@ -10,7 +10,7 @@
 		[codeblock]
 		var interface = XRServer.find_interface("Native mobile")
 		if interface and interface.initialize():
-		    get_viewport().xr = true
+		    get_viewport().use_xr = true
 		[/codeblock]
 	</description>
 	<tutorials>