committers-guide: Increase subject length limit to 67 characters

I originally suggested a 63-character limit "so that git log --oneline
avoids wrapping." Short hashes in the tree are typically 12 characters,
and a space between the hash and the subject should result in a limit
of 67 characters. I think I arrived at 63 by subtracting git's typical
4-space indentation from 67, but that indent is not applicable to
git log --oneline.

Reviewed by: olce
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48246