How do I change the subject in the new site welcome email

I have set the set up so users can create their own site, however when they receive the email below I would like to edit the " Subject :New WordPress Site:
Can you tell me were I can do that?

Thanks
Subject :New WordPress Site:
Dear User,

Your new site has been successfully set up at:
http://asdgfasdg.mysite.com/

You can log in to the administrator account with the following information:
Username: asdgfasdg
Password: UAew4MlqnV7i
Log in here: http://asdgfasdg.mysite.com/wp-login.php

We hope you enjoy your new site. Thanks!

--The Team @