fix missing test in readme

Author: chenyuntc

chapter6-实战指南/README.md

@@ -41,5 +41,5 @@ python main.py help
 ## 测试
 
 ```
-python main.py --data-root=./data/test --use-gpu=False --batch-size=256
+python main.py test --data-root=./data/test --use-gpu=False --batch-size=256
 ```