docs: Issue on page Installation

[abramche]

I would like to report an issue on page https://python-driver.docs.scylladb.com/stable/installation.html#verifying-your-installation

Problem

Broken syntax on the prod page

python -c 'import cassandra; print cassandra.__version__'

Suggest a fix

Seems to be already correct in master, backport needed (?)

[Lorak-mmk]

Not backport, just a release that actually builds a new docs version - I think we forgot to do that during few past releases, lets not forget now.