Wrong documentation for entity event

Created by: temparus

For the additional_fields of event the documentation / description is wrong. It should be the following:

must be provided in form of a JSON-Schema. Youcan add here fields you want to know from people signing up going further than their email-address. The JSON-Schema will always require these fields: {"$schema": "http://json-schema.org/draft-04/schema#","type": "object","additionalProperties": false}