fix M5Stack Unit M6L build errors

variants/m5stack_unit_c6l/platformio.ini

@@ -5,7 +5,7 @@ board_build.partitions = min_spiffs.csv ; get around 4mb flash limit
 build_flags =
   ${esp32c6_base.build_flags}
   ${sensor_base.build_flags}
-  -I variants/M5Stack_Unit_C6L
+  -I variants/m5stack_unit_c6l
   -D P_LORA_TX_LED=15
   -D P_LORA_SCLK=20
   -D P_LORA_MISO=22