Bug 28688: Add renewal to crontab example

Bug 28688: Add renewal to crontab example

Note: I did not add an example to the debian cron daily file, since
we have no commented lines there. And automatic renewal is no default
behavior.

Test plan:
Read new lines in misc/cronjobs/crontab.example. Just comments.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>