Fix capitalisation in marlin_STM32H723Vx.json to match folder name

This allows building on case-sensitive filesystems

Author: liampwll

buildroot/share/PlatformIO/boards/marlin_STM32H723Vx.json

@@ -6,7 +6,7 @@
     "f_cpu": "550000000L",
     "mcu": "stm32h723vet6",
     "product_line": "STM32H723xx",
-    "variant": "MARLIN_H723vx"
+    "variant": "MARLIN_H723Vx"
   },
   "connectivity": [
     "can",