docs: add filter to StreamUserListController (#1267)

The filter is a required field for the API call

docusaurus/docs/Flutter/stream_chat_flutter_core/stream_user_list_controller.mdx

@@ -27,6 +27,7 @@ class UserListPageState extends State<UserListPage> {
   /// [StreamUserListController].
   late final userListController = StreamUserListController(
     client: StreamChatCore.of(context).client,
+    filter: Filter.notEqual('id', StreamChatCore.of(context).currentUser!),
   );
 ```
 
@@ -109,4 +110,4 @@ Widget build(BuildContext context) => Scaffold(
     );
 ```
 
-In this case we're using the [`LazyLoadScrollView`](./lazy_load_scroll_view.mdx) widget to load more data when the user scrolls to the bottom of the list.
+In this case we're using the [`LazyLoadScrollView`](./lazy_load_scroll_view.mdx) widget to load more data when the user scrolls to the bottom of the list.