Fix typo in new setting description

Summary:
Noticed by @amckinley in
https://secure.phabricator.com/D18850#inline-57246 but not fixed
before landing.

Test Plan: ispell

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, amckinley, epriestley

Differential Revision: https://secure.phabricator.com/D18861