Update machine_learning.md

[keep9oing]

Previous manual doesn't activate the conda environment after create it.
The (base) and (tensorflow) prefix should be written.

[ROBOTIS-David]

@keep9oing

Hi, I am sorry that I have missed this request, and thank you for your helping as well.
I reported this request to the one who is in charge of TurtleBot3.

Thank you.

@ROBOTIS-Will
I wonder if you could simply review the file requested from our sophisticated user?

and let me know if I need to update it

Thank you.

[ROBOTIS-Will]

@keep9oing
Thanks for the PR.
Creating and running ROS on conda environment can cause python interpreter issues such as rospkg import error, but we'll see if there's a better way to adopt your PR into our eManual.
https://answers.ros.org/question/282982/how-importerror-no-module-named-rospkg-happens/

[bluedaze]

Is there plans on updating this?