Fix up start, monitor baud (#20326)

Marlin/src/MarlinCore.cpp

@@ -982,7 +982,7 @@ void setup() {
     serial_connect_timeout = millis() + 1000UL;
     while (!MYSERIAL1 && PENDING(millis(), serial_connect_timeout)) { /*nada*/ }
   #endif
-  SERIAL_ECHO_MSG("start");
+  SERIAL_ECHOLNPGM("start");
 
   #if BOTH(HAS_TFT_LVGL_UI, MKS_WIFI_MODULE)
     mks_esp_wifi_init();