URI vs URI-reference

[n3wscott]

Found an issue when URI was changed to be URI-reference, the definition of URI was removed but the usage of URI remains in schemaurl.

see https://github.com/cloudevents/spec/blob/master/spec.md#schemaurl for usage of URI

[cneijenhuis]

Good catch, that was me in #338.

I think I started looking for usages of URI in the Swagger/JSON schema - which doesn't contain schemaurl at the moment - and, as I only found source, changed only that in spec.md.

I'll clean that up, and also add schemaurl to spec.json.

Thanks!

[cneijenhuis]

Correction: schemaurl has already been added to spec.json yesterday via #372 😉

Opened #386