manuals: Fix dates

These were reported by mandoc -T lint ... as warnings.

Signed-off-by: Graham Percival <gperciva@tarsnap.com>
Reviewed by: mhorne
MFC after: 3 days
Sponsored by: Tarsnap Backup Inc.
Pull Request: https://github.com/freebsd/freebsd-src/pull/1475

(cherry picked from commit b74aaa1a2199261f9078247d29481a994b6b5e42)