Fix typo in PhabricatorUserPreferencesSearchEngine
Summary: Looks like a very old typo that prevented visiting `/settings/` on an install.
Test Plan:
1. Visit before this diff and get "PhabricatorSettingApplication is not installed."
2. Visit after this diff and get a functioning, if useless, search controller.
Reviewers: aklapper, O1 Blessed Committers
Reviewed By: aklapper, O1 Blessed Committers
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26323