webhook: merge request event example is incorrect

Problem to solve

The example merge request event is wrong concerning the assignee object: https://docs.gitlab.com/ee/user/project/integrations/webhooks.html#merge-request-events

The assignee is shown to be included in "object_attributes" but it belongs to the root object (tested on gitlab.com v11.10.4-ee)

{
   "object_kind":"merge_request",
   "event_type":"merge_request",
   "user":{ },
   "project":{ },
   "object_attributes":{ },
   "labels":[ ],
   "changes":{ },
   "repository":{ },
   "assignee":{
      "name":"atn nurbel",
      "username":"atnurbel",
      "avatar_url":"https://secure.gravatar.com/avatar/db61f09eaf464f65781eadb618562df7?s=80\\u0026d=identicon"
   }
}

Proposal

The documentation should be modified to show where the assignee is actually sent in the json message.