hier.7: Add /dev/gpt
While here, fix a typo.
MFC after: 3 days
Reviewed by: mhorne, emaste, ziaee
Approved by: mhorne (mentor)
Pull Request: https://github.com/freebsd/freebsd-src/pull/1608
(cherry picked from commit f47cbb29e1c2bcb5b5ad838d2d5342a47b0c4692)