Adding migration to change making-schools-and-colleges... slug

The content of this policy has been changed so the slug needs to be changed to update this. This is Pivotal ticket https://www.pivotaltracker.com/story/show/83664248. Thanks to @issyl0 for the code.
This won't work because only the first `policy_document_slug` is of `document_type` "Policy", the other two will be "SupportingPage".