Wrong example in Issue and MR webhhoks examples

Problem to solve

The example of Issue event and Merge request event are wrong comparing to what happens at run time.

Further details

When I play with master on my computer and I fire an issue or a merge request event webhook, I have this json :

{
  "object_kind":"issue",
  "event_type":"issue",
  "user":{...},
  "project":{...},
  "object_attributes":{...},
  "labels":[...],
  "changes":{
    "updated_at":{
      "previous":"2019-04-06 21:20:45 UTC",
      "current":"2019-04-06 21:21:08 UTC"
    },
    "updated_by_id":{
      "previous":null,
      "current":50
    },
...
}

But in the doc there is :

Proposal

I'll update the doc in the MR following this issue creation.

Who can address the issue

Anyone, maybe someone able to exec a webhook!?

Other links/references

I have updated go-gitlab lib but author don't want my PR because of the doc :D https://github.com/xanzy/go-gitlab/pull/608/commits/02a6db52b192d793eba24b9aab4ca87f7ec773c2