Fixing chapter 5 answers - 3.b.ii, 5.b, 5.c, 5.d

Just fixing a few answers in chapter 5:

3.b.ii - LOOCV uses k=n folds, rather than k=1 folds. (See page 181 of the book.)

5.b, 5.c, 5.d - An error in the "return" statement was causing the test error rates to be way off. As a gut check, the "default" variable is "No" 96.67% of the time, so any reasonable strategy would have an error rate of 3.33% or less, which matches my assertions about the test error rates.

Thanks!
Kevin