Fix m.call.negotiate schema and example

[zecakeh]

Introduced in #1511. The properties and example look like a copy-paste of m.call.invite, but don't match the description of the event, nor MSC2746.

cc @dbkr

Preview: https://pr1546--matrix-spec-previews.netlify.app

[zecakeh]

Furthermore it looks like this paragraph in the description is out of place. It probably deserves to be in the text of the module, and/or on the corresponding events/fields description:

matrix-spec/data/event-schemas/schema/m.call.negotiate.yaml

Lines 30 to 35 in bd6644b

	In the `m.call.invite` and `m.call.answer` events, the `offer` and `answer`
	fields respectively are objects of type `RTCSessionDescriptionInit`. Hence
	the `type` field, whilst redundant in these events, is included for ease of
	working with the WebRTC API and is mandatory. Receiving clients should not
	attempt to validate the `type` field, but simply pass the object into the
	WebRTC API.

And given the text in it, I'm wondering if we should remove the enum property of the corresponding schemas, so clients don't try to validate them against the given values.