doc: python: fixing mistake in venv example
doc: python: fixing mistake in venv example

When updating the section to python 3 some places still
referred to pythonPackages and were overlooked.
Decided to switch it to be more similar to the first
example binding pythonPackages and clarified comments a
bit based on confusion I observed on IRC.

Related to https://github.com/NixOS/nixpkgs/pull/77569

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