Its always recommended to edit the cron file using the command
and not directly edit the file in the spool directory.
NOTE: Chances are there may not be any file inside the cron directory if there are no cron jobs defined before.
The crontab entry by tuxianD is correct.
So, the steps would be:
1- crontab -e
2- Add the cron job (0 16 * * 1 /bin/tar czf /backup/boot_files.cron /boot)
3- Save and exit (:wq)