Update cron.md

[mox1]

crontab entries need a user. When copying and pasting the examples directly into my crontab, they were not running. Using a crontab checking utility, an error was generated:

Checking correctness of /var/spool/cron/crontabs/root

*/5 * * * * /usr/local/bin/autorestic -v -c /root/.autorestic.yml --ci cron |& /usr/bin/logger -t autorestic > /dev/null 2>&1 e: INVALID_USER: Invalid username "/usr/local/bin/autorestic" E: There were 1 errors and 0 warnings.

This update adds a username (root) to the example, which fixes the syntax error.