I haven't found any supported (or unsupported) method for changing subscription emails; they seem to be pretty static. I recommend searching our Ideas section to see if anyone else has had the same idea. If not, create a new idea and encourage others to vote the idea up.
I'm also moving this question to our Tableau Server Administration section where other users may have a better idea.
Thanks for posting,
+1 on Diego's answer, there's no known customization for that email notification.