change the code-block header of the docs
change the code-block header of the docs

https://docs.openstack.org/sahara/wallaby/user/plugins.html#managing-plugins
cannot show the code block, try to change with *code-block*.

Change-Id: I3bff505d48a817cb96ddd0055102e35faaf4b97d

openstack/sahara,openstack/sahara