nanobsd.8: Chase usage string updates

Update the manual page for NanoBSD to match its usage.

Reviewed by: emaste, imp
Approved by: emaste (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D47505

(cherry picked from commit f6f311d8cdacdc964967e83c8f6377d8465806d5)