Replace last 'reboot' occurrences with 'shutdown -r now'

The patch that fixes reboot issues is only in the upcoming FreeBSD-16
and will not be merged back to 15, 14, or 13 versions. Hence, use the
'shutdown -r now' instead of 'reboot' consistently throughout the
handbook.

PR: 293519
Differential Revision: https://reviews.freebsd.org/D56631