Update IRI for RDF 1.2 XML Syntax recommendation in all examples

[jaw111]

As mentioned by @gkellogg on w3c/rdf-turtle#113

The example either needs other updates to be consistent with the current RDF/XML state (e.g., https://www.w3.ort/TR/rdf12-xml and editors), or it stay they way it was historically.

So, if the title in examples is "RDF 1.2 XML Syntax", the IRI should be changed from http://www.w3.org/TR/rdf-syntax-grammar to https://www.w3.org/TR/rdf12-xml/ in all figures, examples, and explanatory text throughout the document.

[jaw111]

As part of the change, could consider aligning the examples to the JSON-LD embedded in the HTML document itself. Main change here would be to use terms from Schema.org vocabulary.