readme: updated formatterPath for nix fmt

[lriesebos]

nix fmt -- - does not seem to work with nixpkgs-fmt. nix fmt -- -- seems to work fine with nixpkgs-fmt and nixfmt-rfc-style.

closes #413

[jnoortheen]

is there any documentation available for the arguments to nix fmt --?

[eclairevoyant]

https://nix.dev/manual/nix/2.18/command-ref/new-cli/nix3-fmt is as much as I've seen

[jnoortheen]

Thanks @lriesebos !

[lriesebos]

ah sorry for the late response, I was out last week. but I see the change was already merged!

the -- is an "end of command options" command. it is used in case there is ambiguity. for example if you have a file called -v then cat -- -v is needed to signal that -v is not the option but a file. for nix fmt, it seems that the first -- signals that the following arguments are passed to the underlying command used by nix fmt (i.e. your outputs.formatter). the second -- is passed to the formatter and seems to be needed to ensure some formatters go to stdin mode which is used by this plugin to format files. I think that is the gist.