Update the comments of stack size for main threads

We updated the comments of stack size for main threads.

Author: TomoYamanaka

rtos/rtx/TARGET_CORTEX_A/RTX_Conf_CA.c

@@ -72,7 +72,7 @@
 
 //   <o>Main Thread stack size [bytes] <64-4096:8><#/4>
 //   <i> Defines stack size for main thread.
-//   <i> Default: 200
+//   <i> Default: 4096
 #ifndef OS_MAINSTKSIZE
  #define OS_MAINSTKSIZE 4096
 #endif