Typo fix for MBED_APP_SIZE

Author: LMESTM

targets/TARGET_STM/TARGET_STM32WB/TARGET_STM32WB55xG/device/TOOLCHAIN_ARM_MICRO/stm32wb55xx.sct

@@ -34,7 +34,7 @@
 #endif
 
 #if !defined(MBED_APP_SIZE)
-; 812KB FLASH
+; 768KB FLASH
 #define MBED_APP_SIZE 0xC0000
 #endif