Adjusting for rdflib.Graph.serialize interface change in 6.0

- Caused by RDFLib/rdflib#1183

docs/adding_data.rst

@@ -152,7 +152,7 @@ using contexts. The code below shows how to do that::
 
    >>> # Serialize the data in the nquads format so we can see that all of our
    >>> # statements are in the proper context
-   >>> print(g.serialize(format='nquads').decode('UTF-8'))
+   >>> print(g.serialize(format='nquads', encoding='UTF-8').decode('UTF-8'))
    <http://example.org/BDW/entities/Widget#12> <http...> <http://example.org/data/imports/BDW_Widgets_2017-2018> .
    <http://example.org/BDW/entities/Widget#12> <...