Fix missing type for semantic ids

Author: andreas-schilling

core/sds-aspect-model-aas-generator/src/main/java/io/openmanufacturing/sds/aspectmodel/aas/PropertyMapper.java

@@ -21,6 +21,7 @@
 import org.eclipse.digitaltwin.aas4j.v3.model.Key;
 import org.eclipse.digitaltwin.aas4j.v3.model.KeyTypes;
 import org.eclipse.digitaltwin.aas4j.v3.model.Reference;
+import org.eclipse.digitaltwin.aas4j.v3.model.ReferenceTypes;
 import org.eclipse.digitaltwin.aas4j.v3.model.SubmodelElement;
 import org.eclipse.digitaltwin.aas4j.v3.model.impl.DefaultKey;
 import org.eclipse.digitaltwin.aas4j.v3.model.impl.DefaultReference;
@@ -125,7 +126,7 @@ default Reference buildReferenceToConceptDescription( final Property property )
                   .type( KeyTypes.CONCEPT_DESCRIPTION )
                   .value( determineIdentifierFor( property ) )
                   .build();
-      return new DefaultReference.Builder().keys( key ).build();
+      return new DefaultReference.Builder().type( ReferenceTypes.EXTERNAL_REFERENCE ).keys( key ).build();
    }
 
    /**