[OVN] DOC Migration Strategy: Adjust formatting in code-block
[OVN] DOC Migration Strategy: Adjust formatting in code-block

Change code-block sections in OVN Migration Strategy, so they
are not solid black rectangles with no visible text.

Trivialfix

Change-Id: I796f9b48e301675db9416c197c765fb09fc9fa80

mahak/neutron,mahak/neutron,openstack/neutron,mahak/neutron,openstack/neutron,openstack/neutron