Add created hook to custom directives code example (#1054)

Author: connectkushal

src/guide/migration/custom-directives.md

@@ -52,6 +52,7 @@ The final API is as follows:
 
 ```js
 const MyDirective = {
+  created() {}, // new
   beforeMount(el, binding, vnode, prevVnode) {},
   mounted() {},
   beforeUpdate() {}, // new