fix: dbpedia URL to use https instead of http (#2444)

The URL for the service keyword had the http address for the dbpedia endpoint, which no longer works. Changing it to https as that works.

docs/intro_to_sparql.rst

@@ -140,7 +140,7 @@ The ``SERVICE`` keyword of SPARQL 1.1 can send a query to a remote SPARQL endpoi
         """
         SELECT ?s
         WHERE {
-          SERVICE <http://dbpedia.org/sparql> {
+          SERVICE <https://dbpedia.org/sparql> {
             ?s a ?o .
           }
         }