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