docs: Update references to JSON RFC

RFC8259 obsoletes RFC7159. Fix a couple of URLs to point to the
newer version.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20181203175702.128701-1-eblake@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>

docs/devel/qapi-code-gen.txt

@@ -26,7 +26,7 @@ how the schemas, scripts, and resulting code are used.
 == QMP/Guest agent schema ==
 
 A QAPI schema file is designed to be loosely based on JSON
-(http://www.ietf.org/rfc/rfc7159.txt) with changes for quoting style
+(http://www.ietf.org/rfc/rfc8259.txt) with changes for quoting style
 and the use of comments; a QAPI schema file is then parsed by a python
 code generation program.  A valid QAPI schema consists of a series of
 top-level expressions, with no commas between them.  Where

docs/interop/qmp-spec.txt

@@ -32,7 +32,7 @@ following format:
 Where DATA-STRUCTURE-NAME is any valid JSON data structure, as defined
 by the JSON standard:
 
-http://www.ietf.org/rfc/rfc7159.txt
+http://www.ietf.org/rfc/rfc8259.txt
 
 The server expects its input to be encoded in UTF-8, and sends its
 output encoded in ASCII.