fix(readme): use attributes of the nix-vscode-extensions attrset because it contains only relevant attributes

Author: deemp

README.md

@@ -101,7 +101,7 @@ If you use NixOS, Home Manager, or similar:
 
 1. (Optional) Allow unfree packages (see [Unfree extensions](#unfree-extensions)).
 
-1. Use `pkgs.vscode-marketplace`, `pkgs.open-vsx` and others (see [`extensions` attrsets](#extensions-attrsets), [example](https://github.com/maurerf/nix-darwin-config/blob/0f88b77e712f14e3da72ec0b640e206a37da7afe/flake.nix#L131)).
+1. Use `pkgs.nix-vscode-extensions.vscode-marketplace`, `pkgs.nix-vscode-extensions.open-vsx` and others (see [`extensions` attrsets](#extensions-attrsets), [example](https://github.com/maurerf/nix-darwin-config/blob/0f88b77e712f14e3da72ec0b640e206a37da7afe/flake.nix#L131)).
 
 > [!NOTE]
 > See [With-expressions](https://nix.dev/manual/nix/latest/language/syntax#with-expressions).