hier(7): tweak /media description

It comes as an empty directory by default. While here, use a serial
(Oxford) comma, per the FDP Primer.

PR: 261349
Reported by: karels
Reviewed by: grahamperrin, karels
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40774

(cherry picked from commit acdf666147eea8bf8626d3733087861493d90e23)