Fix Incorrect DNS Zone API Specifications

The spec for the /getDnsZone and /getDnsZones API calls say that they return JSON where the value for key "domain" is a string. However, these calls return a JSON object for the key "domain".

I ran into this problem when trying to use the go client library and this inconsistency with spec and implementation means that JSON parsing the response fails.

This issue can be reproduced with the following curl commands:

/getDnsZones

curl -H "Authorization: Bearer <personal_access_token>" https://api.netlify.com/api/v1/dns_zones/

/getDnsZone

curl -H "Authorization: Bearer <personal_access_token>" https://api.netlify.com/api/v1/dns_zones/<zone_id>/

The response I receive from these two API calls has the following type of JSON object for the "domain" key:

  "domain": {
    "id": string
    "name": string
    "user_id": string
    "created_at": string
    "updated_at": string
    "expires_at": string
    "registered_at": string
    "renewal_price": number
    "auto_renew": bool
    "auth_code": ??? (I got null),
    "transferred_at": ??? (I got null)
    "auto_renew_at": string
  }

Use the /getDnsZone and /getDnsZones operations as entry points, then locate their response schemas in the OpenAPI specification. Compare the documented domain field with the object returned by the provided curl requests and update the schema so the generated Go client can parse it. Done means both operations describe domain consistently with the observed response.