docs: change term reference to italicized
For `:term:` to work there must be a corresponding glossary error, this
is currently resulting in a warning and just being passed through as
plain text, changing it to italicized as that seems more appropriate.
@@ -10,8 +10,8 @@ Functional properties
---------------------
Use :meth:`~rdflib.graph.Graph.value` and
-:meth:`~rdflib.graph.Graph.set` to work with :term:`functional
-property` instances, i.e. properties than can only occur once for a resource.
+:meth:`~rdflib.graph.Graph.set` to work with *functional
+property* instances, i.e. properties than can only occur once for a resource.
.. code-block:: python
