fix wrong dc terms namespace prefix in codelist SKOS export

Tested with UNECE - UNTDID 1001 - Document name code.

V4 export has @prefix ns0: <dc:> . while V3 has @prefix dc: <http://purl.org/dc/terms/>

Both are wrong!

It should be the same as the DCAT export on project level:

@prefix dcterms: <http://purl.org/dc/terms/> .