fixed nullable

Author: Guy Davenport

Specification/BrAPI-Genotyping/VariantSets/Schemas/VariantSet.yaml

@@ -97,6 +97,6 @@ components:
         referenceSetDbId:
           description: The ID of the reference set that describes the sequences used by the variants in this set.
           type: string
-          nullable: tru
+          nullable: true
           example: 57eae639