[go_router] docs: fix flutter run command for Stateful Nested Navigation (#7158)

There was a typo or out-of-date reference to a dart file in the go router example of Stateful Nested Navigation.

This PR fix the name of the file in the `flutter run` command.

Resolve [issue 151961](flutter/flutter#151961)

Author: se09deluca

packages/go_router/example/README.md

@@ -31,7 +31,7 @@ An example to demonstrate how to use redirect to handle a synchronous sign-in fl
 An example to demonstrate how to use handle a sign-in flow with a stream authentication service.
 
 ## [Stateful Nested Navigation](https://github.com/flutter/packages/blob/main/packages/go_router/example/lib/stateful_shell_route.dart)
-`flutter run lib/stacked_shell_route.dart`
+`flutter run lib/stateful_shell_route.dart`
 
 An example to demonstrate how to use a `StatefulShellRoute` to create stateful nested navigation, with a
 `BottomNavigationBar`.
@@ -49,4 +49,4 @@ An example to demonstrate how to use a complex object as extra.
 ## [Books app](https://github.com/flutter/packages/blob/main/packages/go_router/example/lib/books)
 `flutter run lib/books/main.dart`
 
-A fully fledged example that showcases various go_router APIs.
+A fully fledged example that showcases various go_router APIs.