fix PopupMenuButton example tag (#97763)

packages/flutter/lib/src/material/popup_menu.dart

@@ -931,8 +931,7 @@ typedef PopupMenuItemBuilder<T> = List<PopupMenuEntry<T>> Function(BuildContext
 /// If both are null, then a standard overflow icon is created (depending on the
 /// platform).
 ///
-/// {@tool snippet}
-///
+/// {@tool dartpad}
 /// This example shows a menu with four items, selecting between an enum's
 /// values and setting a `_selectedMenu` field based on the selection
 ///