clean Alternatives section in README

[KATANAND]

I think we should remove serokell/nixfmt link from # Alternatives as it now just redirects to NixOS/nixfmt

https://github.com/NixOS/nixfmt/blob/master/CHANGELOG.md#revision-history-for-nixfmt:

• This project graduated from a Serokell project to an official Nix project, with a repository under the NixOS org and a community-based Nix formatting team as maintainers.

https://github.com/NixOS/nixfmt?tab=readme-ov-file#acknowledgements:

nixfmt was originally developed by Serokell and later donated to become an official Nix project with the acceptance of RFC 166.

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Also nix-community/nixpkgs-fmt was archived and the project's README states that it was "Replaced by NixOS/nixfmt" so maybe it is also a candidate for removal?

Edit: fixed wrong link