ccdconfig.8: Refer to graid(8) and zfs(8) instead of gvinum(8)

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D48536

(cherry picked from commit 86116ab256c476191e2fc1fffd79f94382960362)
(cherry picked from commit 8432ddac21b3da5fa45b4f934e90e80deaa9cabf)