python@3.12: further tweak EXTERNALLY-MANAGED guidance

[mikebeaton]

This updates the very welcome additions in 6e68ac8, but incorporates two further changes:

• I believe there is an error in the previous source env/bin/activate, which should read source path/to/venv/bin/activate, given the context
• The previous guidance (prior to 6e68ac8) mentioned python3 instead of python. AFAICT this is more likely to work (e.g. if Python 2 is also installed) and using python in these instructions might perhaps cause additional unnecessary confusion in this case, so this proposes using python3 again

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[woodruffw]

These changes make sense to me, thank you @mikebeaton!

FWIW there is no more default python for Python 2 on macOS anymore, although Homebrew also doesn't do the python3 -> python alias by default. So this change makes sense, but I want to qualify the right reason for it 🙂

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @mikebeaton!