Fix error in gmountver(8) man page

Also fixes the usage statement for the binary to use the correct terms (provider and geom name, rather than dev and prov, which is incorrect in the latter case)
Sync the man page summary with the new usage statement

PR: 199540
Differential Revision: https://reviews.freebsd.org/D2329
Submitted by: Fabian Keil
Reviewed by: trasz
Approved by: eadler (mentor)
Obtained from: ElectroBSD (original)
MFC after: 1 week
Sponsored by: ScaleEngine Inc.