doc: replace macos install command with immediately runnable one

[corneliusroemer]

The following should work for most people (at least it did for me) and saves command line editing need.

From pipx --help:

--python PYTHON       Python to install with. Possible values can be the executable name (python3.11), the version of an available system
                        Python or to pass to py launcher (3.11), or the full path to the executable. Requires Python 3.8 or above.