Correct comment on watchdog timer

Marlin/watchdog.cpp

 //=================================== ISR ===================================
 //===========================================================================
 
-// Watchdog timer interrupt, called if main program blocks >1sec and manual reset is enabled.
+// Watchdog timer interrupt, called if main program blocks >4sec and manual reset is enabled.
 #if ENABLED(WATCHDOG_RESET_MANUAL)
   ISR(WDT_vect) {
     SERIAL_ERROR_START;