fix document preview command's directory parameter missing problem on…

… python3

docs/contributing.md

@@ -109,7 +109,7 @@ Before opening a pull request, you can preview your changes if you have set up t
 
 or if you only have Python 3 installed:
 
-    python3 -m http.server 8936
+    python3 -m http.server 8936 --directory docs
 
 and navigating to `http://localhost:8936/`.