Update custom-directive.md (vuejs#1198) (vuejs#397)

potato4d · re-fort · commit 3be93b316c9a · 2017-10-10T14:13:31.000Z
I suggest add this params demo because i can not understand when i saw this part,i think other people will have the same confusion
diff --git a/src/v2/guide/custom-directive.md b/src/v2/guide/custom-directive.md
@@ -44,6 +44,9 @@ Vue.directive('focus', {
 directives: {
   focus: {
     // ディレクティブ定義
+    inserted: function (el) {
+      el.focus()
+    }
   }
 }
 ```
