Update user_extensions.md (#2162)

stop() {
        this.eventBus.removeListenersExtension(this);
    }

Is simply wrong. Changing to 

    stop() {
        this.eventBus.removeListeners(this);
    }

seems to do the trick...

docs/advanced/more/user_extensions.md

@@ -46,7 +46,7 @@ class MyExampleExtension {
      * Is called once the extension has to stop
      */
     stop() {
-        this.eventBus.removeListenersExtension(this);
+        this.eventBus.removeListeners(this);
     }
 }