Included correct python version.

example_13.md

@@ -26,13 +26,13 @@ Now we are going to use a node to send a a move base goal half a meter in front
 ```bash
 # Terminal 2
 cd catkin_ws/src/stretch_tutorials/src/
-python navigation.py
+python3 navigation.py
 ```
 
 ### The Code
 
 ```python
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import rospy
 import actionlib
@@ -114,9 +114,9 @@ if __name__ == '__main__':
 Now let's break the code down.
 
 ```python
-#!/usr/bin/env python
+#!/usr/bin/env python3
 ```
-Every Python ROS [Node](http://wiki.ros.org/Nodes) will have this declaration at the top. The first line makes sure your script is executed as a Python script.
+Every Python ROS [Node](http://wiki.ros.org/Nodes) will have this declaration at the top. The first line makes sure your script is executed as a Python3 script.
 
 ```python
 import rospy