Replace old setting reference in documentation (#791)
Replace old setting reference in documentation (#791)

`FORMAT_KEYS` has been removed in version 3.0 and only
setting `JSON_API_FORMAT_FIELD_NAMES` is now available.
leo-naeka/django-rest-framework-json-api,django-json-api/django-rest-framework-json-api,django-json-api/rest_framework_ember,django-json-api/django-rest-framework-json-api