Fix in basic_usage.rst (#168)

* Fix  in basic_usage.rst

You have to be inside mace dir to run git fetch

docs/user_guide/basic_usage.rst

@@ -19,6 +19,7 @@ Here we use the mobilenet-v2 model as an example.
     .. code:: sh
 
         git clone https://github.com/XiaoMi/mace.git
+        cd mace/
         git fetch --all --tags --prune
 
         # Checkout the latest tag (i.e. release version)