Fix error message for `annotate_overrides`.
Fix error message for `annotate_overrides`.

BUG=
R=brianwilkerson@google.com

Review URL: https://codereview.chromium.org//1748353003 .

dart-lang/linter,dart-lang/linter,alexeieleusis/dart-lang_linter,alexeieleusis/dart-lang_linter