mount.8: Add a single example for single user mode

The zfs command to do this is a bit longer and harder to remember. In
the last few releases mount(8) learned how to do this, so lets show it
in the manual.

MFC after: 3 days
Reported by: Jan Bramkamp <crest+freebsd@rlwinm.de>
Discussed with: cperciva, emaste
Reviewed by: mhorne
Approved by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D49988