policykit dtd is not available for validation

Currently, the documentation describes the XML header for policy files as:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE policyconfig PUBLIC "-//freedesktop//DTD PolicyKit Policy Configuration 1.0//EN"
"http://www.freedesktop.org/standards/PolicyKit/1.0/policyconfig.dtd">

However, http://www.freedesktop.org/standards/PolicyKit/1.0/policyconfig.dtd does not exist on the freedesktop.org servers. That causes tools that do dtd validation to fail.