Fix typo

Summary: This typo appeared in the docs and I thought I'd fix it

Test Plan: See that the typo at src/applications/config/option/PhabricatorCoreConfigOptions.php is gone

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25496