Update docs.

Author: quic-zhanweiw

docs/python.md

@@ -23,6 +23,7 @@ git clone https://github.com/quic/ai-engine-direct-helper.git
 ```
 If you have cloned it before, you can update the code by the following command:
 ```
+cd ai-engine-direct-helper
 git pull --recurse-submodules
 ```
 ### Step 4: Setup QAI AppBuilder Python Environment: