Use 'quota' command in disk space section.

This is in the vein of our "show, don't tell" theme. There is a command to show quotas, so instead of having to update it in the manual whenever it changes, we can kill two birds with one stone by showing students how to use the command.

My only concern is that it might be a little technical for the Rules and Procedures section, which at the moment contains no other technical information.

Please discuss.