updated cron example to current owncloud path

Old:
````
    # crontab -u www-data -e
    */15  *  *  *  * php -f /var/www/htdocs/owncloud/cron.php
````
New:
````
  # crontab -u www-data -e
  */15  *  *  *  * php -f /var/www/owncloud/cron.php
````