Find line 109 in sendnotice.php and change it to:
Content visible to verified customers only.
It will now say:
Ben, your ad on Our Forum for " FIAT Spyder " is going to expire in X days.
( I couldn't think of a better way to word it so it would fit all ads, but you can obviously change to suit your needs - just make sure the $title text stays as it is - and if you want to use " (speech marks) in the email, make sure you put a backslash (\) before them - as seen in the example above)