R1:5ae3af6691ca

Fix an outdated HTML anchor link in Diffusion table of contents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…