use correct key looking for a public author of a task

Test Plan:
Used the scripts/mail_handler.php with and without patch and saw
the maniphest task being created with patch applied.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley, davidreuss

Differential Revision: 1041

src/applications/metamta/storage/receivedmail/PhabricatorMetaMTAReceivedMail.php

@@ -71,7 +71,7 @@ class PhabricatorMetaMTAReceivedMail extends PhabricatorMetaMTADAO {
         $this->setAuthorPHID($user->getPHID());
       } else {
         $default_author = PhabricatorEnv::getEnvConfig(
-          'metamta.manipest.default-public-author');
+          'metamta.maniphest.default-public-author');
 
         if ($default_author) {
           $user = id(new PhabricatorUser())->loadOneWhere(