Change policies to use document_page class

The class for policies seem to have been accidentally changed
from document_page to document-page in
46170bfe2853406a56fcb204e05a38ea942777cf .
Fix styling on document collections

document-page class was added in
46170bfe2853406a56fcb204e05a38ea942777cf . In theory, this page
should probably be a document-page, but this needs refactoring
at some point.
Merge pull request #1101 from alphagov/61610260-fix-document-page-styling

Fix styling of policies and collections

https://www.pivotaltracker.com/s/projects/367813/stories/61610260