Update installation_guide.md

Author: mattia-gramuglia

installation_guide.md

@@ -73,13 +73,13 @@ Now you can try to run the demos to check if the installation is correct.
 For example,
 ```bash
 cd mbs/
-python3.9 demos_MBS_revolute.py
+python demos_MBS_revolute.py
 ```
 ## Step 6: Try to run the code
 Clone the repo and navigate to the project folder.
 ```bash
 git clone https://github.com/andrealaffly/UAV_Sim_PyChrono.git
 cd UAV_Sim_PyChrono
 git checkout dev
-python3.9 main.py
-```
+python main.py
+```