Update README.md

README.md

@@ -43,7 +43,7 @@ So far, the RT-Thread scons building system support the command line compiling o
 
 In rtconfig.py file:
 
-* ```RTT_CC``` the compiler which you want to use, gcc/keil/iar. 
+* ```CROSS_TOOL``` the compiler which you want to use, gcc/keil/iar. 
 * ```EXEC_PATH``` the path of compiler. 
 
 In SConstruct file: