git-cl: Fix error message when -B is not specified.

Bug: 1018293
Change-Id: I9b915c3b4a1218e6318efde005c4f38243a35055
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1900309Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

git_cl.py

@@ -428,7 +428,7 @@ def _get_bucket_map(changelist, options, option_parser):
   if options.bucket:
     return {options.bucket: {b: [] for b in options.bot}}
   option_parser.error(
-      'Please specify the bucket, e.g. "-B luci.chromium.try".')
+      'Please specify the bucket, e.g. "-B chromium/try".')
 
 
 def _parse_bucket(raw_bucket):