Fix ST STM32G0316-DISCO flash size (#767)

Author: MalteP

boards/disco_g031j6.json

@@ -25,7 +25,7 @@
   "name": "ST STM32G0316-DISCO",
   "upload": {
     "maximum_ram_size": 8192,
-    "maximum_size": 131072,
+    "maximum_size": 32768,
     "protocol": "mbed",
     "protocols": [
       "blackmagic",