Updates to conform with RFC 7159.

src/json.m:
	RFC 7159 specifies that JSON text may consist of any valid JSON
	value, not just an array or object.  Delete the get_text/4 predicate
	which was only present for conformance with RFC 4627.

	s/JOSN/JSON/ in a spot.

	Fix some documentation layout.

README:
	Update the RFC number.