Fix grammar in Route doc comment (#63132)

packages/flutter/lib/src/widgets/navigator.dart

@@ -109,7 +109,7 @@ enum RoutePopDisposition {
 /// platform-adaptive transition.
 ///
 /// A route can belong to a page if the [settings] are a subclass of [Page]. A
-/// page-based route, as opposite to pageless route, is created from
+/// page-based route, as opposed to a pageless route, is created from
 /// [Page.createRoute] during [Navigator.pages] updates. The page associated
 /// with this route may change during the lifetime of the route. If the
 /// [Navigator] updates the page of this route, it calls [changedInternalState]