Docs reference a flake output which does not exist

[erooke]

From the readme:

To create an environment with CadQuery and python-language-server (where hopefully your IDE will pick up python-language-server and supply autocomplete, docs, etc.):
```sh
nix shell github:marcus7070/cq-flake#cadquery-env
```

However this throws an error when it is run:

$ nix shell github:marcus7070/cq-flake#cadquery-env
error: flake 'github:marcus7070/cq-flake' does not provide attribute 'packages.x86_64-linux.cadquery-env', 'legacyPackages.x86_64-linux.cadquery-env' or 'cadquery-env'