Fix header formatting typo in PopupMenuButton docs (#139084)

I believe this was intended (in #128351) to be a heading, but currently in the [published docs](https://api.flutter.dev/flutter/material/PopupMenuButton-class.html) it is rendered as:

 ```
/// ## Updating to MenuAnchor
```

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

@@ -1047,7 +1047,7 @@ typedef PopupMenuItemBuilder<T> = List<PopupMenuEntry<T>> Function(BuildContext
 /// If both are null, then a standard overflow icon is created (depending on the
 /// platform).
 ///
-/// /// ## Updating to [MenuAnchor]
+/// ## Updating to [MenuAnchor]
 ///
 /// There is a Material 3 component,
 /// [MenuAnchor] that is preferred for applications that are configured