add one liner

Author: guilyx

README.md

@@ -60,8 +60,7 @@ If you want to use it by cloning the project:
 $ git clone https://github.com/APLA-Toolbox/PythonPDDL
 $ cd PythonPDDL
 $ python3 -m pip install -r requirements.txt
-$ git submodule init
-$ git submodule update
+$ git submodule update --init
 ```
 
 You should have a `pddl-examples` folder containing PDDL instances.