[NUCLEO_F302R8] Fix wrong clock define used by RTX

Author: dbestm

libraries/rtos/rtx/TARGET_CORTEX_M/RTX_Conf_CM.c

@@ -160,7 +160,7 @@
 #    define OS_CLOCK       168000000
 
 #elif defined(TARGET_STM32F302R8)
-#    define OS_CLOCK       64000000
+#    define OS_CLOCK       72000000
 
 #elif defined(TARGET_STM32L053R8) || defined(TARGET_STM32L053C8)
 #    define OS_CLOCK       32000000