doc: remove explicit libpython3.8-dev from Ubuntu deps

Having libpython3.8-dev explicitly mentioned causes issues if
"python3-dev" pulls a Python version that's not 3.8. What's more,
python3-dev already pulls the correct "versionless" libpython3-dev
anyway.

Fixes #66461

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>