handbook: fix example for vlan

The VLAN name "Camera(s)" is plural in the ifconfig cli command
but singular in the rc.conf example.

PR: 243986
Submitted by: wjsmyth@gmail.com
Patch by: wjsmyth@gmail.com
Approved by: bcr@(mentor)
Differential Revision: https://reviews.freebsd.org/D23606