Update inputs.md

local file uri's have `file://` and then absolute paths start with `/` so it's three slashes. .... some time later ... Oh, this may have been a relative path. Those are problematic: https://github.com/NixOS/nix/issues/12281, but maybe here is not the place to say so. I'm not sure if a change is actually warranted, I was just confused. Sorry for the noise. Feel free to close this, I figure I'll leave it in the current state as an FYI.