Docs Python: fix example on how to override the package set

Not passing in the newly created interpreter as `self` results in an
incorrect `passthru`. Solves #64334.

(cherry picked from commit f1ff85e)

doc/languages-frameworks/python.section.md

@@ -635,7 +635,7 @@ with import <nixpkgs> {};
         };
       });
     };
-  in pkgs.python3.override {inherit packageOverrides;};
+  in pkgs.python3.override {inherit packageOverrides; self = python;};
 
 in python.withPackages(ps: [ps.blaze])).env
 ```