Clarify the definition of isomorphism and caon

[aidhog]

• "without changing the topology of the graph." -> "generating an RDF dataset R' such that R' = S" I think that the "topology of the graph" is not defined elsewhere. Proposing a replacement that is still concise and easy to understand (I hope) but more precise.
• "is a dataset R_C such that [R ≈ R_C and] R_C = S_C if and only if R ≈ S" Adding that the the canonical form should be isomorphic to the original graph. (This is not covered by the latter condition.)

[iherman]

Thanks @aidhog