Enhancement: 20_Open_The_Screen.ipynb

[dotnetdork]

This update converts the 'Create codespace on master' from an image to a span that is contained within the markdown; it also offers readability improvements, clearer wording, and a new image example.

https://github.com/dotnetdork/Python-Apprentice/blob/typos/lessons/10_Turtles/10_Welcome/20_Open_The_Screen.ipynb