Fix Ultimaker servo pin

As pointed out at https://github.com/MarlinFirmware/Marlin/issues/7981#issuecomment-407116642

Marlin/src/pins/pins_ULTIMAKER.h

@@ -43,7 +43,7 @@
 //
 // Servos
 //
-#define SERVO0_PIN         13   // UNTESTED
+#define SERVO0_PIN         11
 
 //
 // Limit Switches