main: add -serial=rtt flag as possible option

I added this a while ago but forgot to change the help documentation.

main.go

@@ -1412,7 +1412,7 @@ func main() {
 	gc := flag.String("gc", "", "garbage collector to use (none, leaking, conservative)")
 	panicStrategy := flag.String("panic", "print", "panic strategy (print, trap)")
 	scheduler := flag.String("scheduler", "", "which scheduler to use (none, tasks, asyncify)")
-	serial := flag.String("serial", "", "which serial output to use (none, uart, usb)")
+	serial := flag.String("serial", "", "which serial output to use (none, uart, usb, rtt)")
 	work := flag.Bool("work", false, "print the name of the temporary build directory and do not delete this directory on exit")
 	interpTimeout := flag.Duration("interp-timeout", 180*time.Second, "interp optimization pass timeout")
 	var tags buildutil.TagsFlag