Remove Dockerfile mention from flake.nix - #5892

When the Dockerfile was removed in 2452dc7 , it looks like the reference to it in the nix flake was forgotten.

Note: There is a second issue with the build, which is related to NixOS/nixpkgs#524985 and probably just needs a nix flake update, I'll investigate that as well in a separate pr.

I've tested this fix by building the package in my nixos configuration, using the following input configuration, with a just-updated lockfile:

My nixpkgs is also on nixpkgs-unstable.

    tree-sitter = {
      url = "github:abeforgit/tree-sitter/fix/remove-dockerfile-mention";
      inputs.nixpkgs.follows = "nixpkgs";
    };

fixes: #5891

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