Fixing wrong pin to heater extruder 2 <-> 3

Extruder 2 and 3 was swapped

Marlin/pins_MEGATRONICS_3.h

 #define KILL_PIN -1
 
 #define HEATER_0_PIN 2
-#define HEATER_1_PIN 8
-#define HEATER_2_PIN 9
+#define HEATER_1_PIN 9
+#define HEATER_2_PIN 8
 #define HEATER_BED_PIN 10
 #define FAN_PIN 6
 #define FAN2_PIN 7