FWIW, that's the one system email related to a membership site you CAN edit.
Go to Pages > Membership. Then click the membership site you wish to edit (for MOST people it's the only site you'll see in this list).
Then scroll down to MESSAGES and click on the one called "MEMBERSHIP NAME - Register".
Click on it and a menu appears. Choose edit.
You'll see this (couldn't add the screenshot here for some reason).
Edit away.
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