Fix an outdated HTML anchor link in Diffusion table of contents

Summary:
See https://discourse.phabricator-community.org/t/navigating-to-changed-files-in-diffusion-does-not-work-anymore/433.

In D18465, I updated these but this hard-coded the anchor for some reason (???) and I missed it while grep-ing.

Test Plan: Viewed a commit (/rXYZaaaa) and clicked a file link in the table of contents. Got modern #change-... anchor and navigation into the document.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18609