horizon quickstart referencing wrong directory path

[Lin Hua Cheng]

In the "horizon quickstart" (horizon/docs/source/quickstart.rst), it provided a sample command for starting the server using manage.py found in "openstack-dashboard" directory.

There are two issues here:
1. All references to openstack-dashboard, should be updated to openstack_dashboard directory.
2. manage.py is not in openstack_dashboard.

The sample command:
$ tools/with_venv.sh dashboard/manage.py runserver
$ tools/with_venv.sh dashboard/manage.py runserver 0.0.0.0:8080

Should be updated to:
$ tools/with_venv.sh ./manage.py runserver
$ tools/with_venv.sh ./manage.py runserver 0.0.0.0:8080

[Tom Fifield (fifieldt)]

Not sure if this should be sent to horizon instead of openstack-manuals, but I've attempted a fix anyway: https://review.openstack.org/#/c/7658