Encoding Property Object: headers property should me a map of [String -> Header (not Header*s*)]

1. [Encoding Property Object](https://github.com/OAI/OpenAPI-Specification/blob/OpenAPI.next/versions/3.0.md#encoding-property-object)#headers	is currently defined as a Map of [String -> Header*s* Object].
2. [Headers Object](https://github.com/OAI/OpenAPI-Specification/blob/OpenAPI.next/versions/3.0.md#headers-object) is already a map of [{name} -> Header Object].

So, current description is equivalent to a map whose values are maps : [String -> [{name} -> Header Object]]. It contradicts to [this Encoding Object example](https://github.com/OAI/OpenAPI-Specification/blob/OpenAPI.next/versions/3.0.md#encoding-object-example)

@timburks and I found this issue while working on [JSON Schema for RC2](https://github.com/googleapis/gnostic/commit/02acf4809ebe82fe6280d952ec7b6467635e5a50#commitcomment-22252403)