Fix TEMP_BED_PIN for Mightyboard Rev.E

Based on #8685

Marlin/pins_MIGHTYBOARD_REVE.h

@@ -123,7 +123,7 @@
 //
 // Temperature Sensors
 //
-#define TEMP_BED_PIN        69   // K7
+#define TEMP_BED_PIN        15   // K7 - 69 / ADC15 - 15
 
 // SPI for Max6675 or Max31855 Thermocouple
 // Uses a separate SPI bus