http_async_client: doc review (payload setting)

src/modules/http_async_client/doc/http_async_client_admin.xml

@@ -331,7 +331,8 @@ t_newtran();
 http_async_query("http://example.com/test.php?r_uri=$rU&f_uri=$fU", "HTTP_REPLY");
 ...
 # POST
-http_async_query("http://example.com/test.php", "{'r_uri':'$rU', 'f_uri':'$fU'}", "HTTP_REPLY");
+$var(payload) = "{'r_uri':'$rU', 'f_uri':'$fU'}";
+http_async_query("http://example.com/test.php", "HTTP_REPLY");
 ...
 route[HTTP_REPLY] {
     if ($http_ok) {