docs: Use nodeDependencies instead of shell.nodeDependencies

The only difference between `shell.nodeDependencies` and
`nodeDependencies` in this example is that `shell.nodeDependencies`
skips #205 - but in the
example given I think it's most likely that people only want the
dependencies to depend on package{,-lock}.json.

README.md

@@ -203,7 +203,7 @@ Here's an example derivation showing this technique:
 
 ```nix
 let
-  nodeDependencies = (pkgs.callPackage ./default.nix {}).shell.nodeDependencies;
+  nodeDependencies = (pkgs.callPackage ./default.nix {}).nodeDependencies;
 in
 
 stdenv.mkDerivation {