[FIX] mail: remove comma

The email content was a tuple instead of a proper string.
Introduced at 0028c421

Fixes tw:jaynvora/status/834435897554710528

addons/mail/models/mail_thread.py

@@ -898,7 +898,7 @@ class MailThread(models.AbstractModel):
 <p>Hello,</p>
 <p>The following email sent to %s cannot be accepted because this is a private email address.
    Only allowed people can contact us at this address.</p>
-</div><blockquote>%s</blockquote>""" % (message.get('to'), message_dict.get('body')),
+</div><blockquote>%s</blockquote>""" % (message.get('to'), message_dict.get('body'))
 
         # Wrong model
         if model and model not in self.env: