[go_router] Fix a typo in the ShellRoute docstring (flutter#3378)

[go_router] Fix a typo in the ShellRoute docstring

Author: emesterhazy

packages/go_router/lib/src/route.dart

@@ -323,8 +323,8 @@ class GoRoute extends RouteBase {
 /// A route that displays a UI shell around the matching child route.
 ///
 /// When a ShellRoute is added to the list of routes on GoRouter or GoRoute, a
-/// new Navigator that is used to display any matching sub-routes, instead of
-/// placing them on the root Navigator.
+/// new Navigator is used to display any matching sub-routes instead of placing
+/// them on the root Navigator.
 ///
 /// To display a child route on a different Navigator, provide it with a
 /// [parentNavigatorKey] that matches the key provided to either the [GoRouter]