Update nix-language.md

[Carlos-Marques]

description example: (e.g. file.nix)

is different from code example: nix-instantiate --eval test.nix --arg pkgs 'import <nixpkgs> {}'

this is not consistent with the rest of the document and the change should avoid some minor mistakes

[fricklerhandwerk]

Thanks a lot, good catch! Happy to see someone is reading it carefully.