image/tarexport: fix error formatting

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

image/tarexport/load.go

@@ -146,7 +146,7 @@ func (l *tarexporter) setParentID(id, parentID image.ID) error {
 		return err
 	}
 	if !checkValidParent(img, parent) {
-		return fmt.Errorf("image %v is not a valid parent for %v", parent.ID, img.ID)
+		return fmt.Errorf("image %v is not a valid parent for %v", parent.ID(), img.ID())
 	}
 	return l.is.SetParent(id, parentID)
 }