Add created hook to custom directives code example (#382)

* Add created hook to custom directives code example

vuejs/docs@4d43757

* Update custom-directives.md

vuejs/docs@8101503

Author: naokie

src/guide/migration/custom-directives.md

@@ -53,7 +53,8 @@ Vue.directive('highlight', {
 
 ```js
 const MyDirective = {
-  beforeMount(el, binding, vnode, prevVnode) {},
+  created(el, binding, vnode, prevVnode) {}, // new
+  beforeMount() {},
   mounted() {},
   beforeUpdate() {}, // 追加
   updated() {},