Fix two small typos in exercise C.3-3

[appleby]

Pr{X = 1} is listed twice when describing the probability density function. The second occurrence is meant to be Pr{X = 0}.

Also, the Pr{X = 2} is given as 3(6/216) - 3(1/126) = 5/216, where 5/216 should actually be 15/216.

In both cases, the correct values already appear in the "calculation" section near the bottom.