doc: update nixpkgs reference

The autotrash package has been moved out of python3Packages, and is also
available in the stable release.

README.md

@@ -28,7 +28,7 @@ Using a distribution package is preferred as it will also ensure that autotrash
         ```
     - On ArchLinux, there is an [AUR package available](https://aur.archlinux.org/packages/autotrash/)
 
-    - On NixOS (or any system with `nix`), there is the `python3Packages.autotrash` package in nixpkgs-unstable
+    - On NixOS (or any system with `nix`), there is the `autotrash` package
 
 Finally you could try using `pip` directly if `pipx` is not available.