docs: use "::" instead of ".. code-block::"

docs: use "::" instead of ".. code-block::"

The former is a short hand for the latter and is already widely used in
the docs. Using the short hand avoids incompatibility with the alternate
impl of rst2html5.

Reviewed-by: Erik Skultety <eskultet@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>