man: Cross-reference zfs-load-key(8) for ENCRYPTION mention

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Submitted-by: Harry Schmalzbauer
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10866