changing sample_ontology_link required fields

Author: slebras

spec/collections/samples/sample_ontology_link.yaml

@@ -3,7 +3,7 @@ type: edge
 schema:
   "$schema": http://json-schema.org/draft-07/schema#
   type: object
-  required: [from, to, created, expired, sample_metadata_term]
+  required: [_from, _to, created, expired, sample_metadata_term]
   description: minimum necessary terms for sample -> ontology edge link.
   properties:
     _from: