The `pythonPackages` definition is redundant

1. Use the same approach like in the overlay example:
   Override `python` instead of `pythonPackages` so that
   `python.pkgs` refers to the new package set like `pythonPackages`.

   This also fixes a bug in the original example where
   `pkgs.fetchgit` was not in scope.

   Add an extra example to illustrate how to override just a
   package set.

2. Fix mix-up between `super` and `self` in the explanation text.
   Also, simplify the explanation.

Use example package `zerobin` instead of `bepasty-server` which
is no longer part of python-packages.
This fixes the examples for current nixpkgs versions.