Fix warning when compiling the doc, title had an underline too short

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>

doc/configuration.rst

@@ -1016,7 +1016,7 @@ Defaults to: ``None``
 
 
 SMTP_CERTFILE
-^^^^^^^^^^^^
+^^^^^^^^^^^^^
 
 This configuration key allows to specify a certificate file to be used in
 the `starttls` command when connecting to the smtp server.