Fix typo in Spaces documentation

Summary: Fixed typo of the word "policy"

Test Plan: Google: "define policy" and see spelling

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Projects: #spaces

Differential Revision: https://secure.phabricator.com/D13263