Update the GELI handbook to be more accurate

The description of the key file incorrectly identified it as the master key

PR: 208578
Submitted by: Wout Decre <wout@canodus.be> (original version)
Reviewed by: bcr, oshogbo
Sponsored by: Klara Inc.
Event: July 2020 Bugathon
Differential Revision: https://reviews.freebsd.org/D15866