doc/python: update buildPythonApplication example

It's recommended to consume the interpreter, not the package
set when being called from `all-packages.nix`

doc/languages-frameworks/python.section.md

@@ -995,18 +995,18 @@ called with `callPackage` and passed `python` or `pythonPackages` (possibly
 specifying an interpreter version), like this:
 
 ```nix
-{ lib, python3Packages }:
+{ lib, python3 }:
 
-python3Packages.buildPythonApplication rec {
+python3.pkgs.buildPythonApplication rec {
   pname = "luigi";
   version = "2.7.9";
 
-  src = python3Packages.fetchPypi {
+  src = python3.pkgs.fetchPypi {
     inherit pname version;
     sha256 = "035w8gqql36zlan0xjrzz9j4lh9hs0qrsgnbyw07qs7lnkvbdv9x";
   };
 
-  propagatedBuildInputs = with python3Packages; [ tornado_4 python-daemon ];
+  propagatedBuildInputs = with python3.pkgs; [ tornado python-daemon ];
 
   meta = with lib; {
     ...