Update instructions for instructor list

Yes, sorry. From Eventbrite:

1. Go to https://www.eventbrite.com/myevents/.
1. Click "Manage" for the appropriate event.
1. Click "Event Reports" from the sidebar.
1. Select "Sales By Ticket Type" under "Report Type".
1. Select "Attended" under "Attendee Status". (This means they checked in.)
1. Select "Teaching Assistant Signup" under "Ticket Type".
1. Click "CSV" button.