Here I did as the guide and got an error return
user:~/catkin_ws$ catkin build
The build space at ‘/home/user/catkin_ws/build’ was previously built by ‘catkin_make’. Please remove the build space or pick a different build space.
So I used the catkin build command inside the my_example_pkg dir
user:~/catkin_ws/src/my_example_pkg$ catkin build
[build] Summary: All 1 packages succeeded!
user:~/catkin_ws/src/my_example_pkg$ source devel/setup.bash
user:~/catkin_ws/src/my_example_pkg$ rosrun my_examples_pkg simple_topic_publisher.py
[rospack] Error: package ‘my_examples_pkg’ not found