Improve example

example.js

@@ -28,6 +28,7 @@ validateValue(data, {
 	one: [ required, isString ],
 	two: [ required, isNumber ],
 	three: [ required, isBoolean ],
+	optional: [ isString ],
 	array: [ required, arrayOf([ required, isString ]) ],
 	mapping: [ required, anyProperty({
 		key: [ required ],