Correct typo: security.alter[n]ate-file-domain
Summary: Minor correction to correct spelling of alternate (the 'n' was missing).
Test Plan: reviewer to verify correct spelling
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: epriestley
Differential Revision: https://secure.phabricator.com/D10175