(stable) Fix PHPMailer/SMTP configuration typo for legacy SMTP configurations

Summary: See https://discourse.phabricator-community.org/t/phpmailer-cannot-read-configuration-value-because-of-typo/1094.

Test Plan: Browsed a dictionary. (This doesn't get hit when configured via cluster.mailers, which is how I previously re-tested SMTP configuration.)

Differential Revision: https://secure.phabricator.com/D19058