[go_router] Fix a typo in a comment in the shell_route.dart (#8235)

Fixed a typo in a comment in the shell_route.dart.

Author: ultraon

packages/go_router/example/lib/shell_route.dart

@@ -87,7 +87,7 @@ class ShellRouteExampleApp extends StatelessWidget {
             },
             routes: <RouteBase>[
               // The details screen to display stacked on the inner Navigator.
-              // This will cover screen A but not the application shell.
+              // This will cover screen C but not the application shell.
               GoRoute(
                 path: 'details',
                 builder: (BuildContext context, GoRouterState state) {