FIX: Add missing custom label header name

sample-management-custom/caosdb-server/scripting/bin/sample_helpers/default_constants.yml

@@ -17,6 +17,7 @@
 #
 csv_column_names:
   entity_id: "LinkAhead ID"
+  custom_label_prop: "Custom Label"
 
 csv_column_descriptions:
   LinkAhead ID: "An ID generated by LinkAhead (either integer or URL to this entity). Do not change this column!"