added the second part of exercise 4.4.2 (b)

[ggrandi]

I was working through the exercise in section 4.4 and in 4.4.2 (b), the part about What about if it is allowed to take positive rational values instead of natural numbers? doesn't have an exercise associated.

I have added a new def Rat.pos_infinite_descent with the same form as Int.infinite_descent to add it as an exercise.

[teorth]

Thanks!