Auth - fix headers for instructions on Phabricator OAuth provider

Summary: Ref T887. Derp'd this in D11287, despite taking screenshots and everything.

Test Plan: looks bettah and more consistent

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T887

Differential Revision: https://secure.phabricator.com/D11294