R1:a8cc1c05e528

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…