test_that: fix typo in usage string. BUG=None TEST=test_that --help shows no typo. Change-Id: I78c1b3619095842c38a4abeb39c4acb95850dbed Reviewed-on: https://chromium-review.googlesource.com/195500 Commit-Queue: Jorge Lucangeli Obes <jorgelo@chromium.org> Tested-by: Jorge Lucangeli Obes <jorgelo@chromium.org> Reviewed-by: Aviv Keshet <akeshet@chromium.org>