Multiply WATCH_TEMP_PERIOD by 1000UL (PR#2535)

instead of 1000.  Else we get errors with times over 32 seconds.

Marlin/temperature.cpp

@@ -1013,7 +1013,7 @@ void tp_init() {
   void start_watching_heater(int e) {
     if (degHotend(e) < degTargetHotend(e) - (WATCH_TEMP_INCREASE + TEMP_HYSTERESIS + 1)) {
       watch_target_temp[e] = degHotend(e) + WATCH_TEMP_INCREASE;
-      watch_heater_next_ms[e] = millis() + WATCH_TEMP_PERIOD * 1000;
+      watch_heater_next_ms[e] = millis() + WATCH_TEMP_PERIOD * 1000UL;
     }
     else
       watch_heater_next_ms[e] = 0;