ZSON not strictly a superset of JSON

[mccanne]

Thanks to Timothy Green for catching this bug in the ZSON spec per a recent tweet https://twitter.com/TRiG_Ireland/status/1538325276253380608

It says that all JSON documents are valid ZSON documents, and then says that ZSON must be UTF-8.

JSON allows UTF-8, UTF-16LE, UTF-16BE, UTF-32LE, UTF-32BE.

I think we should keep the UTF-8 limitation and update the sentence claiming that ZSON is a superset of JSON with respect to the text encoding.