[Docs] Better example

docs/api/entities/user.pug

@@ -101,7 +101,7 @@ block content
 				"banner_id": "584bfc82d8e5186f8f755ec5",
 				"banner_url": "https://file.himasaku.net/584bfc82d8e5186f8f755ec5",
 				"bio": "どうすれば君だけのために生きていけるの",
-				"birthday": null,
+				"birthday": "1997-12-06",
 				"created_at": "2016-09-07T13:46:56.605Z",
 				"drive_capacity": 1073741824,
 				"email": null,