Copy paste error in print statement for cross-validation of KNN

[covuworie]

There is a copy-paste typo in the last cell of the Lecture_6_Notebook.ipynb. The value of k is being chosen, not the degree of the polynomial. It instead should read something like:

print('the test R^2 for a {} knn model is: {}'.format(best_k, test_R_sq))

PS: I didn't submit a pull request as you probably don't want the output from my notebook.