Small fixes

generic.py

@@ -526,7 +526,7 @@ def get_latest_comment(yaml_object):
         else:
             return score_obj['comment']
     else:
-        return empty
+        return ''
 
 
 def get_latest_date(yaml_object):

technique_mapping.py

@@ -125,7 +125,7 @@ def _write_layer(layer, mapped_techniques, filename_prefix, name):
 def _layer_metadata_make_compliant(metadata):
     """
     Make sure the metadata values in the Navigator layer file are compliant with the expected data structure
-    https://github.com/mitre-attack/attack-navigator/blob/master/layers/LAYERFORMATv2_2.md
+    from the latest version on: https://github.com/mitre-attack/attack-navigator/tree/master/layers
     :param metadata: list of metadata dictionaries
     :return: compliant list of metadata dictionaries
     """