Correct ADC Vref divisor for MKS TinyBee

MKS TinyBee uses a 2.5V VDDA ADC reference instead of the more common 3.3V.  2500.0 = 2.5V.
Board uses a Zener diode - measuring the actual voltage on any disconnected thermistor pin and using that value ( x 1000) will improve precision.

Marlin/src/HAL/ESP32/HAL.cpp

@@ -240,8 +240,12 @@ void MarlinHAL::adc_start(const pin_t pin) {
   const adc1_channel_t chan = get_channel(pin);
   uint32_t mv;
   esp_adc_cal_get_voltage((adc_channel_t)chan, &characteristics[attenuations[chan]], &mv);
-  adc_result = mv * 1023.0 / 3300.0;
-
+  #if MB(MKS_TINYBEE)
+    adc_result = mv * 1023.0 / 2500.0; // TinyBee uses 2.5v VDDA ref
+  #else
+    adc_result = mv * 1023.0 / 3300.0;
+  #endif
+
   // Change the attenuation level based on the new reading
   adc_atten_t atten;
   if (mv < thresholds[ADC_ATTEN_DB_0] - 100)