docs: update UPnP gdscript snippet to latest gdscript

This commit adds usage of the new (stringless) signal API.

modules/upnp/doc_classes/UPNP.xml

@@ -31,13 +31,13 @@
 
 		    if err != OK:
 		        push_error(str(err))
-		        emit_signal("upnp_completed", err)
+		        upnp_completed.emit(err)
 		        return
 
 		    if upnp.get_gateway() and upnp.get_gateway().is_valid_gateway():
 		        upnp.add_port_mapping(server_port, server_port, ProjectSettings.get_setting("application/config/name"), "UDP")
 		        upnp.add_port_mapping(server_port, server_port, ProjectSettings.get_setting("application/config/name"), "TCP")
-		        emit_signal("upnp_completed", OK)
+		        upnp_completed.emit(OK)
 
 		func _ready():
 		    thread = Thread.new()