docs/interop/qmp-spec: Update error description for parsing errors

The description text for a parsing error has changed since the
spec doc was first written; update the example in the docs.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20230515162245.3964307-3-peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

docs/interop/qmp-spec.rst

@@ -307,7 +307,7 @@ This section provides some examples of real QMP usage, in all of them
   .. code-block:: QMP
 
     -> { "execute": }
-    <- { "error": { "class": "GenericError", "desc": "Invalid JSON syntax" } }
+    <- { "error": { "class": "GenericError", "desc": "JSON parse error, expecting value" } }
 
 .. admonition:: Example