docs: change term reference to italicized
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.

RDFLib/rdflib,RDFLib/rdflib,RDFLib/rdflib,RDFLib/rdflib