[doc] Fixed formatting of devstack/README.md
[doc] Fixed formatting of devstack/README.md

Fixed headings on local.conf example.

Change-Id: I928e57c1b8e1bc9c357cecfe8b7cfe2f0f3efbc6
Closes-Bug: #1697409

stackforge/monasca-api,openstack/monasca-api,stackforge/monasca-api,stackforge/monasca-api,openstack/monasca-api,stackforge/monasca-api,openstack/monasca-api