use `uncefact` or `unece` consistently

[VladimirAlexiev]

(Related to https://github.com/uncefact/spec-jsonld/issues/184)

The uncefact context is at https://vocabulary.uncefact.org/unece-context.jsonld. But I think the name unece is inconsistent, since the domain name and the ontology use uncefact.

So I suggest to use these URLs for the file downloads:

• https://vocabulary.uncefact.org/uncefact.jsonld: ontology
• https://vocabulary.uncefact.org/uncefact-context.jsonld: context

[VladimirAlexiev]

• the ontology uses uncefact eg "@id": "uncefact:abbreviatedName"
• https://vocabulary.uncefact.org/properties uses unece, eg unece:abbreviatedName

[VladimirAlexiev]

• Now the ontology uses "@id": "unece:abbreviatedName" (consistent)
• The context is at https://vocabulary.uncefact.org/unece-context.jsonld (consistent)
• The ontology is at https://vocabulary.uncefact.org/ (doesn't mention unece) and requires -H 'Accept: application/ld+json'. It would be better to also serve it from https://vocabulary.uncefact.org/unece.jsonld (See #184)

I've changed the issue title to NOT prefer one word over the other.