Fix flake build failure

Ran `nix flake lock --update-input mmdoc`, which fixes the following
build failure:

```
error:
       … while fetching the input 'github:ryantm/nixpkgs/6a6caacfdd079a0fa249046514480a1c4597d861'

       error: unable to download 'https://github.com/ryantm/nixpkgs/archive/6a6caacfdd079a0fa249046514480a1c4597d861.tar.gz': HTTP error 404

       response body:

       404: Not Found
```