Add 'cp' of murano-dashboard lib file in devstack README

This patch https://review.openstack.org/#/c/87751 introduced murano
dashboard in devstack files.

the cp of murano-dashboard has not been added in the README

Change-Id: I3feb15b613f678278b0cecabf1d50c141b8e3e88
Merge "Add 'cp' of murano-dashboard lib file in devstack README"