Add correct ADC_MULTIPLIER definition to Heltec Wireless Paper

variants/heltec_wireless_paper/platformio.ini

@@ -22,6 +22,7 @@ build_flags =
   ;-D PIN_BOARD_SCL=18               ; same GPIO as P_LORA_TX_LED
   -D PIN_USER_BTN=0
   -D PIN_VEXT_EN=45
+  -D ADC_MULTIPLIER=8.4
   -D PIN_VBAT_READ=20
   -D PIN_ADC_CTRL=19
   -D SX126X_DIO2_AS_RF_SWITCH=true