Describe the bug
"Declared by" source links are broken in the NixOS manual's Appendix A. Configuration Options

To Reproduce
Run nixos-help
Click next to go to Appendix A
Click on a source link

Expected behavior
I should land at the appropriate link for my Nixpkgs version, e.g. https://github.com/NixOS/nixpkgs/blob/8746c77a383/nixos/modules/config/appstream.nix

Actual behaviour
I land at a broken link for "release-19.09" e.g.
https://github.com/NixOS/nixpkgs/blob/release-19.09/nixos/modules/config/appstream.nix

Metadata

 - system: `"x86_64-linux"`
 - host os: `Linux 5.2.8, NixOS, 19.09pre188690.8746c77a383 (Loris)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.2.2`
 - channels(craig): `""`
 - channels(root): `"nixos-19.09pre188690.8746c77a383"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`