When you say "the crontab" you're not being specific. You should say which one you mean. However, based on the contents, it looks like
/etc/crontab. Since that file does have a user field, it doesn't make sense that you'd get that error. My bet is that there's a user crontab that has that line in it. User crontabs don't contain user fields.
The proper thing to do is not to change that line in
/etc/crontab. There may be other jobs it runs.
You can edit that file and the files in the
/etc/cron.* subdirectories using any editor. You can specify the editor you want to use when you use
crontab -e by setting the
EDITOR environment variable.
You should use
crontab -e to edit user crontabs (including root's). Remove the user field or remove that line in the user crontab where it appears (not
/etc/crontab). User crontabs don't have a user field, but the system crontabs do.
User crontabs are located in
/var/spool/cron/crontabs. You can
sudo grep for the line to find out which file has the stray line. Again, only use
crontab -u username -e to edit the user crontab.