As Richard mentioned, I also think the email is being sent from Turnitin's side; not Moodle's. I recommend contacting your Turnitin sales rep (who may re-direct you to put in a ticket with their support team) and ask if it's customizable. It might be part of the integration in which case the Turnitin rep should be able to point you to the right spot in the code. No matter the results, let us know what you find out from your Turnitin rep as I'm sure others may look to do the same in the future.
Happy 
oodling!