Description of datatyped literals needs more work

[TallTed]

_Originally posted by @TallTed in https://github.com/w3c/N3/pull/154#discussion_r1131572503_ Lines 493-495 of spec/index.html will need some additional work.

                                                                                             These are represented as a string, 
        followed by the `^^` symbol and the corresponding <dfn data-cite="XMLSCHEMA11-2#built-in-datatypes">datatype IRI</dfn>.
        (`xsd:string` is the datatype IRI for strings; this is the default and can be omitted).

As it stands now, it suggests that "test"^^ would be valid, as the string "test" is followed by ^^ and omits xsd:string (or more precisely, omits http://www.w3.org/2001/XMLSchema#string).

[william-vw]

@TallTed Feel free to propose a PR to fix the issue. Note that this section is non-normative.