examples: changed stacksize to the recommended value

examples/riot_and_cpp/main.cpp

@@ -37,7 +37,7 @@ extern "C" {
 using namespace std;
 
 /* thread's stack */
-char threadA_stack [KERNEL_CONF_STACKSIZE_PRINTF];
+char threadA_stack [KERNEL_CONF_STACKSIZE_MAIN];
 
 /* thread's function */
 void *threadA_func(void *arg);