Fixed namespace in dbpedia_get_wikidata_uris()

[ChristophLeonhardt]

In dbpedia_get_wikidata_uris() there is an option to query additional properties from DBpedia. This is implemented like that:

dbpedia/R/wikidata.R

Line 48 in a1bfc2c

optional <- sprintf('OPTIONAL { ?item dbo:%s ?key . }', optional)

A potential issue which would need to be checked, I guess, is that not all properties necessarily can be queried via the ontology namespace (?), i.e. with dbo:%s. If I look around at the DBpedia page for http://de.dbpedia.org/page/Mannheim, there are other namespaces such as "geo" or "prop-de" which probably would need to be named accordingly in a SPARQL query.

As I am not entirely sure, it should be checked whether I am correct. For example, I would assume that you cannot query latitude ("lat") as an optional attribute right now.