Use better terminology for RDF equivalency

[webb]

In v4.0alpha1 rule 4-2, use better terminology than "equal to" for RDF equivalency.

[webb]

See https://www.w3.org/TR/2014/REC-rdf11-concepts-20140225/#entailment for RDF equiavalence intro.

See https://www.w3.org/TR/2014/REC-rdf11-concepts-20140225/#graph-isomorphism for graph isomorphism.

[webb]

Use Equivalent from the first link above. Define the term and use it in 4-2.