[Mailer] Remove duplicate semicolon

Author: sebpacz

mailer.rst

@@ -324,7 +324,7 @@ header, etc.) but most of the times you'll set text headers::
         ->getHeaders()
             // this header tells auto-repliers ("email holiday mode") to not
             // reply to this message because it's an automated email
-            ->addTextHeader('X-Auto-Response-Suppress', 'OOF, DR, RN, NRN, AutoReply');
+            ->addTextHeader('X-Auto-Response-Suppress', 'OOF, DR, RN, NRN, AutoReply')
 
         // ...
     ;