geli(8): Add missing commands in the EXAMPLES section

• Add a missing 'geli attach' command
• Fix the passphrase prompt for a 'geli attach' command

Reported by: Fabian Keil <freebsd-listen at fabiankeil dot de>
Reviewed by: bcr (mentor)
Approved by: bcr (mentor)
Differential Revision: https://reviews.freebsd.org/D25761