Better example

Don't use {} in one-liners

doc/sphinx/whats-new/upgrading.rst

@@ -53,7 +53,7 @@ The synthetic keyword is now a function::
     if (resp.status == 799) {
         set resp.status = 200;
         set resp.http.Content-Type = "text/plain; charset=utf-8";
-        synthetic({"You are "} + client.ip);
+        synthetic("You are " + client.ip);
         return (deliver);
     }