Fix bad "SMTP" and "cluster.mailers" default value

Summary: See note in D19964.

Test Plan: O_o

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19966

src/applications/config/option/PhabricatorMetaMTAConfigOptions.php

@@ -188,7 +188,7 @@ EODOC
       pht('Configuring Outbound Email')));
 
     return array(
-      $this->newOption('cluster.mailers', 'cluster.mailers', null)
+      $this->newOption('cluster.mailers', 'cluster.mailers', array())
         ->setHidden(true)
         ->setDescription($mailers_description),
       $this->newOption(

src/docs/user/configuration/configuring_outbound_email.diviner

@@ -235,7 +235,7 @@ To use this mailer, set `type` to `sendmail`. There are no `options` to
 configure.
 
 
-Mailer: STMP
+Mailer: SMTP
 ============
 
 You can use this adapter to send mail via an external SMTP server, like Gmail.