fix(doc): update the use of GoRouter in the guides

docusaurus/docs/Flutter/05-guides/09-initialize_stream_chat_widget_tree.mdx

@@ -489,8 +489,10 @@ class _MyAppState extends State<MyApp> {
 
   late final _router = GoRouter(
     initialLocation: '/',
-    navigatorBuilder: (context, state, child) {
-      if (state.location.startsWith('/chat')) {
+    routes: [
+      ShellRoute(
+        builder: (context, state, child) {
+          if (state.uri.host.startsWith('/chat')) {
             wasPreviousRouteChat = true;
             return StreamChat(
               client: _client,
@@ -536,6 +538,8 @@ class _MyAppState extends State<MyApp> {
             ],
           ),
         ],
+      ),
+    ],
   );
 
   @override