buildbucket.config: fix typo for options

CL 559740 tried to hide the retry builds button, but failed, likely
because the 'option' header is supposed to be spelled 'options'. Fix
that now.

Change-Id: I71f5f446a294768df60a88eecc3cb1a18bedb723
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/560195
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>