mixer(8): Update FILES section

While here, change "N" to "X" to match sndctl.8's notation.

Sponsored by: The FreeBSD Foundation
MFC after: 4 days
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D53503

(cherry picked from commit 26e7e9a6a2a2ee8101b0d06c83ac6d66b78a985d)