Typo in Exercise 7.1 (b): reference to Exercise 6.6 should be 6.5

[jy-gh]

From Exercise 7.1 (b):

Note: This part doesn't involve a lot of code, but there are a lot of low-level fiddly bits. The solution will look almost the same as for Exercise 6.6.

There is no Exercise 6.6. The previous exercise was 6.5, not 6.6.

[Curiosit]

Created a pull here: https://github.com/dabeaz-course/python-mastery/pull/65

[dabeaz]

Fixed!