Correct notification.pid to notification.pidfile

Summary:
The correct conf variable is notification.pidfile, not notification.pid.
See also:

• src/applications/config/option/PhabricatorNotificationConfigOptions.php:58
• src/applications/aphlict/management/PhabricatorAphlictManagementWorkflow.php:34

Test Plan:
Grepped the whole source of Phabricator for occurences of notification.pid,
but all matches pointed to notification.pidfile instead.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11768