Fix heat endpoint in install guide

The previous endpoint description was plain wrong.

This patches to use the endpoint as described in:
https://github.com/openstack-dev/devstack/blob/master/files/keystone_data.sh

backport: havana

Change-Id: Ic90bace1e60f3cfddf00dd5594cee5d083b75ae7
Merge "Fix heat endpoint in install guide"