Fix pep8 errors in releasenotes/source/conf.py
Fix pep8 errors in releasenotes/source/conf.py

Change-Id: I514db428850e94d9d12b003780ef5899a28a15bc
Closes-Bug: #1648796

stackforge/networking-ovs-dpdk,openstack/networking-ovs-dpdk,stackforge/networking-ovs-dpdk,openstack/networking-ovs-dpdk,stackforge/networking-ovs-dpdk