Fix use of deprecated API in example (#4414)

Author: TatuLund

src/main/java/com/vaadin/demo/component/menubar/MenuBarDividers.java

@@ -21,7 +21,7 @@ public MenuBarDividers() {
         subMenu.add(new Hr());
         subMenu.addItem("By email");
         subMenu.addItem("Get link");
-        subMenu.add(new Hr());
+        subMenu.addComponent(new Hr());
         subMenu.addItem("Set permissions");
         // end::snippet[]