You left out the "in" before Tk. Also since it is so important to a new GUI programmer to understand that they are dealing with a hidden eventloop, not just some linear code, I think you should emphasize the importance of it more. Some concepts to add:

The MainLoop is tied to the Mainwindow. Kill the loop, kill the window.

Never use sleep in a gui program because it will put the loop to sleep and make the whole window unresponsive.

Other typos: but you knew knew that , Frames are very simple widgets. They are used as a type of border should be "Frames are very simple container widgets; they act as a container for other widgets";

Besides that, you switched to the grid packing manager, from pack, and don't explain it.

Uh.. my eyes are strained already. :-) The last thing I will mention, is you make no mention of the Tk "widget" program, which shows all the widgets in action.