Fixed typo on modal notification (#3619)

tutorials/gui/custom_gui_controls.rst

@@ -181,7 +181,7 @@ exists, but which can be checked with the _notification callback:
             NOTIFICATION_RESIZED:
                 pass # Control changed size; check new size
                 # with get_size().
-            NOTIFICATION_MODAL_CLOSED:
+            NOTIFICATION_MODAL_CLOSE:
                 pass # For modal pop-ups, notification
                 # that the pop-up was closed.