jsonrpc-server: Report monitor session ID properly in error message.

The error message in question is about the monitor session ID but it
actually reports the JSON-RPC request ID instead, which is surprising.

Found by inspection.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Justin Pettit <jpettit@ovn.org>

ovsdb/jsonrpc-server.c

@@ -1415,7 +1415,7 @@ ovsdb_jsonrpc_monitor_cond_change(struct ovsdb_jsonrpc_session *s,
 
     m = ovsdb_jsonrpc_monitor_find(s, params->u.array.elems[0]);
     if (!m) {
-        error = ovsdb_syntax_error(request_id, NULL,
+        error = ovsdb_syntax_error(params->u.array.elems[0], NULL,
                                    "unknown monitor session");
         goto error;
     }