bsdinstall: update comment related to pkg

pkg_add has been gone since 2013(?). Refer to pkg(8) instead.

Sponsored by: The FreeBSD Foundation
Reviewed by: jrtc27
Differential Revision: https://reviews.freebsd.org/D44946

(cherry picked from commit ad31d4764255c0848f1f0fa10760d16b5a2922aa)