Go to Admin panel -> Manage -> Forms
Click on Ticket Details.
to the right of "Issue Details" click on the config button.
In the Help Text field remove the contents and re-type it in.
The default is:
Please fill out Details on the reason(s) for opening the ticket.
click Save.
See if that fixes the issue.
If it does not then you will likely have to edit the database directly to remove it.