nixosConfigurations example does not invoke nixosConfigurations

[makoConstruct]

The example in https://flake.parts/options/flake-parts.html#opt-flake.nixosConfigurations doesn't use nixosConfigurations and so does not demonstrate its use.

[roberth]

According to module system convention, this is how examples are supposed to be written; for instance, you can just put a value there, and it will be rendered to Nix expression literals. However, this is certainly a valid point.

[makoConstruct]

I see. I didn't at all infer, from the title, that the place this value was supposed to be put was nixosConfigurations =, I guess in retrospect I don't know how many readers would have that issue.

But it is generally good to show an example of the thing being used in its usual context.