Update Getting Started so autobuild matches manual build
Update Getting Started so autobuild matches manual build

Default settings in conf.py has `make html` put the output into _build/html and https://github.com/github/gitignore/blob/master/Python.gitignore defaults to ignoring docs/_build/ so the `sphinx-autobuild` command example should output to the same ignored spot.
davidfischer/readthedocs.org,rtfd/readthedocs.org,davidfischer/readthedocs.org,rtfd/readthedocs.org,davidfischer/readthedocs.org,pombredanne/readthedocs.org,safwanrahman/readthedocs.org,safwanrahman/readthedocs.org,rtfd/readthedocs.org,safwanrahman/readthedocs.org,pombredanne/readthedocs.org,davidfischer/readthedocs.org,rtfd/readthedocs.org,safwanrahman/readthedocs.org,pombredanne/readthedocs.org