Make sure edition id is used, not document id

A previous commit (https://github.com/alphagov/whitehall/commit/50851298a3df6fecd9b639680643602e7f07584c)
changed the name of this form field, but forgot to change the fact that
we now want the edition's ID, not its document ID. This corrects that.
Merge pull request #1464 from alphagov/fix-for-relating-detail-guides

Make sure edition id is used, not document id