doc: Update README text about Flutter main channel

We plan to continue to live at head, using the Flutter main channel,
in the future when the app is out of beta.  Update the README to
reflect that.

See 67f8429 (#899) which made a similar change in the comment
in pubspec.yaml.

Author: gnprice

README.md

@@ -117,8 +117,8 @@ see our [full setup guide](docs/setup.md).
 
 ### Flutter version
 
-While in the beta phase, we use the latest Flutter from Flutter's
-main branch.  Use `flutter channel main` and `flutter upgrade`.
+We use the latest Flutter from Flutter's main branch.
+Use `flutter channel main` and `flutter upgrade`.
 
 We don't pin a specific version, because Flutter itself doesn't offer
 a way to do so.  So far that hasn't been a problem.  When it becomes one,