git-arc.1: list width typo

Reviewed by: mhorne
MFC after: 3 days
Fixes: 760be44702 ("git-arc: document "create" command options")
Pull Request: https://github.com/freebsd/freebsd-src/pull/1272

(cherry picked from commit 41d6858ebd6ae0431196c1bc6f03f8e388eae8e6)