Modify 2-byte code of "heap and stack" test main for Cortex-A

I modified "space" character from 2-byte code to 1-byte code in "heap and stack" test main.

Author: TomoYamanaka

TESTS/mbedmicro-rtos-mbed/heap_and_stack/main.cpp

@@ -16,7 +16,7 @@
  */
 
 #if defined(TARGET_CORTEX_A)
-　  #error [NOT_SUPPORTED] This function not supported for this target
+    #error [NOT_SUPPORTED] This function not supported for this target
 #endif
 
 #include <stdio.h>