Fix JSON response

What does this MR do?

Fix response sample for POST /runners. It seems the id is a number, not a string.

POST http://gitlab.example.com/api/v4/runners
Private-Token: gkndLG-tzRsemb4myqa9
Content-Type: application/json; charset=utf-8

{
  "description": "test",
  "token": "1yCWaimpn3nqSKUKGjio"
}

201 Created
Server: nginx
Date: Fri, 08 Jan 2021 21:03:38 GMT
Connection: keep-alive
Cache-Control: must-revalidate, max-age=0, private
ETag: W/"a2feb14ffa6fccb3b71a144625961761"
Vary: Origin
X-Content-Type-Options: nosniff
X-Frame-Options: SAMEORIGIN
X-Gitlab-Feature-Category: continuous_integration
X-Request-ID: 01EVHWZPB4SJPF74JJXVYMSTZ5
X-Runtime: 0.115645
Strict-Transport-Security: max-age=31536000
Referrer-Policy: strict-origin-when-cross-origin
Content-Type: application/json
Content-Length: 39

{
  "id": 4,
  "token": "VaDmXsWxyceGvaqKzchy"
}

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