fix: clarify note about reverting Countdown function

[yemiwebby]

While following the mocking chapter of the book, I ran into a test failure after modifying the Countdown() function as suggested. The issue was that there wasn't an explicit instruction to revert the code snippet to its original, working version afterwards.

This small detail can easily confuse Go beginners who are following along line by line. Although the source code serves as a reference, adding a clear note in the book helps avoid unnecessary confusion and improves the learning experience.

This PR adds that clarification as a note just after the line:

if you run your tests they should still be passing even though the implementation is wrong.

@quii