Fix an outdated HTML anchor link in Diffusion table of contents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