handbook/disks: Update encrypted swap instructions for geli

Add instructions to initialize the geli swap device first. Also, be a
bit clearer on the /etc/fstab line to add for geli.

While here: fix a typo in the Camellia-CBC algorithm.

PR: 282806
Reviewed by: carlavilla@
Event: Halifax Hackathon 202606
Location: Dalhousie CS Faculty building entrance hallway
Differential Revision: https://reviews.freebsd.org/D57904