Add examples to the help string for git-cl-try's -B arg

LUCI bucket can take a few different formats, and it's not clear which
one `git cl try` wants. So this clarifies it a bit.

Bug: None
Change-Id: I4816cf17e3af376d447395021fbc3544616c5451
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3624550
Auto-Submit: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>

git_cl.py

@@ -4670,7 +4670,8 @@ def CMDtry(parser, args):
             'available.'))
   group.add_option(
       '-B', '--bucket', default='',
-      help=('Buildbucket bucket to send the try requests.'))
+      help=('Buildbucket bucket to send the try requests. Format: '
+            '"luci.$LUCI_PROJECT.$LUCI_BUCKET". eg: "luci.chromium.try"'))
   group.add_option(
       '-r', '--revision',
       help='Revision to use for the tryjob; default: the revision will '