Fix Hotend Idle Timeout comment (#18032)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/Configuration_adv.h

  */
 //#define HOTEND_IDLE_TIMEOUT
 #if ENABLED(HOTEND_IDLE_TIMEOUT)
-  #define HOTEND_IDLE_DURATION_SEC    5     // (minutes) Time without extruder movement to trigger protection
+  #define HOTEND_IDLE_TIMEOUT_SEC (5*60)    // (seconds) Time without extruder movement to trigger protection
   #define HOTEND_IDLE_MIN_TRIGGER   180     // (°C) Minimum temperature to enable hotend protection
   #define HOTEND_IDLE_NOZZLE_TARGET   0     // (°C) Safe temperature for the nozzle after timeout
   #define HOTEND_IDLE_BED_TARGET      0     // (°C) Safe temperature for the bed after timeout

Marlin/src/feature/hotend_idle.h

 public:
   static void check();
 private:
-  static constexpr millis_t hp_interval = SEC_TO_MS(HOTEND_IDLE_DURATION_SEC);
+  static constexpr millis_t hp_interval = SEC_TO_MS(HOTEND_IDLE_TIMEOUT_SEC);
   static millis_t next_protect_ms;
   static void check_hotends(const millis_t &ms);
   static void check_e_motion(const millis_t &ms);