doc: Update instructions on upgrading Flutter

The `tools/check` run is largely redundant with Flutter upstream's CI,
which will have run our `flutter analyze` and `flutter test` before
allowing any given upstream change to be merged.  If something gets
past that (perhaps because it affects some other suite like
re-generating our generated files), our own CI will catch it.

Author: gnprice

README.md

@@ -255,8 +255,7 @@ To update the version bounds:
 * Use `flutter upgrade` to upgrade your local Flutter and Dart.
 * Run `tools/upgrade flutter-local`, which makes a commit updating
   `pubspec.yaml` and `pubspec.lock` to match your local Flutter.
-* Make a quick check that things work: `tools/check`,
-  and do a quick smoke-test of the app.
+* Build and run the app for a quick smoke-check.
 * Send the changes as a PR.