Fixing test runner `--help` typo
Fixing test runner `--help` typo

Change-Id: I2fc4bfc8f9f43ae379cfc590c4dc4d6765a493de
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/239732
Commit-Queue: Mark Zhou <654f5fe0c4ca6c799b0e735dcf3fc1c389f35bf9@google.com>
Reviewed-by: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@google.com>
Commit-Queue: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk