python setup.py install --show-default-install-dirs: Don't say "install"

[jidanni]

docs/documentation.html says

You can see a list of the default installation locations by running:

# python setup.py install --show-default-install-dirs

Please just say to do

# python setup.py --show-default-install-dirs

As we are not trying to seemingly install anything, but just show things. So no need for the whole scary line.

(Sure, I could fix the code directly,
but one cannot tell if documentation.html is automatically produced from documentation.txt, or visa versa.
Or maybe you are maintaining them both by hand.)