Fixed M5Stack build.board name (espressif#1187)

Author: M5StackJimmyLai

boards.txt

@@ -1218,7 +1218,7 @@ m5stack-core-esp32.serial.disableRTS=true
 m5stack-core-esp32.build.mcu=esp32
 m5stack-core-esp32.build.core=esp32
 m5stack-core-esp32.build.variant=esp32
-m5stack-core-esp32.build.board=M5Stack-Core-ESP32
+m5stack-core-esp32.build.board=M5Stack_Core_ESP32
 
 m5stack-core-esp32.build.f_cpu=240000000L
 m5stack-core-esp32.build.flash_size=4MB