test_push: Fix help message.

After struggling to figure out the format of the value expected by
--build, fix the help message for the next person.

BUG=None
TEST=None

Change-Id: Ic3051c4ef58ca942acf7bd5984baf57d613325d0
Reviewed-on: https://chromium-review.googlesource.com/348790
Commit-Ready: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Dan Shi <dshi@google.com>
diff --git a/site_utils/test_push.py b/site_utils/test_push.py
index ebeba82..03fc60b 100755
--- a/site_utils/test_push.py
+++ b/site_utils/test_push.py
@@ -162,7 +162,7 @@
     parser.add_argument('-i', '--build', dest='build', default=None,
                         help='Default is the latest canary build of given '
                              'board. Must be a canary build, otherwise AU test '
-                             'will fail.')
+                             'will fail. (ex: gandolf-release/R53-8397.0.0)')
     parser.add_argument('-si', '--shard_build', dest='shard_build', default=None,
                         help='Default is the latest canary build of given '
                              'board. Must be a canary build, otherwise AU test '