Update README.md doc build instructions
Update README.md doc build instructions

This updates the sphinx-build command example in README.md
to match the output path of `python setup.py build_sphinx`.

Change-Id: I7940f2a7f9f3ca0436620058684c0eb4b4f3427a

ionrock/designate,muraliselva10/designate,ionrock/designate,cneill/designate,ionrock/designate,kiall/designate-py3,openstack/designate,cneill/designate,openstack/designate,tonyli71/designate,grahamhayes/designate,ramsateesh/designate,kiall/designate-py3,tonyli71/designate,kiall/designate-py3,kiall/designate-py3,cneill/designate,cneill/designate,ramsateesh/designate,grahamhayes/designate,cneill/designate-testing,muraliselva10/designate,cneill/designate-testing,muraliselva10/designate,openstack/designate,grahamhayes/designate,cneill/designate,kiall/designate-py3,cneill/designate-testing,tonyli71/designate,ramsateesh/designate