git-for-each-ref.txt: 'raw' is a supported date format

Commit 7dff9b3 (Support 'raw' date format) added a raw date format.
Update the git-for-each-ref documentation to include this.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>