fix api uage

Author: gtfierro

python/example.py

@@ -40,5 +40,5 @@
 
 # get an rdflib.Dataset (https://rdflib.readthedocs.io/en/stable/apidocs/rdflib.html#rdflib.Dataset)
 ds = env2.to_rdflib_dataset()
-for graph in list(ds.contexts()):
+for graph in list(ds.graphs()):
     print(f"Graph {graph.identifier} has {len(graph)} triples")