Fix driver docstring RST formatting
Fix driver docstring RST formatting

The docstring for the driver was using the ".. code::" directive
instead of the ".. code-block::" directive, causing some issues
with doc parsing.

Change-Id: Ib4e23c855ba14ed925644aa12173ac9d3deb34ba

openstack/cinder,phenoxim/cinder,mahak/cinder,phenoxim/cinder,mahak/cinder,openstack/cinder