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!),
   );
 ```