Correct @id return value from r_navigation

The `@id` key in the response from r_navigation is actually the url path to the current navigation request. Here I have changed the return for `@id` to the objectId and moved the url path that was in `@id` to a new `dts:references` key. This seems to make more sense to me but perhaps it doesn't conform to the standard.

capitains_nautilus/apis/dts.py

@@ -469,7 +469,8 @@ def r_dts_navigation(self, objectId=None, passageId=None, start=None, end=None,
  },
  "level": references.level,
  "passage": url_for(".dts_document", id=objectId, _external=self._external)+"{&ref}{&start}{&end}",
- "@id": url_for(".dts_navigation", **params),
+ "@id": objectId,
+ "dts:references": url_for(".dts_navigation", **params),
  "hydra:member": [_ref_to_dict(ref) for ref in references]
  }
  if references.citation: