doc: Replace Python 2 package names with Python 3 equivalents

librados-intro.rst referenced ``python-rados`` for CentOS/RHEL.
rbd-openstack.rst referenced ``python-rbd`` for both apt and yum.
Python 2 reached end-of-life in January 2020; these package names
install the Python 2 bindings (or fail entirely) on current distros.
Replace with the correct Python 3 package names: python3-rados and
python3-rbd.

Fixes: https://tracker.ceph.com/issues/77200

Signed-off-by: Emmanuel Ameh <eameh@contractor.linuxfoundation.org>