Typo (5.1.3.2)

[kosgdk]

5.1.3.2 Working With Generic Types
Typo in code in part 2, 3 and 4:

val example = Pair(1, Pair(2, Pair(3, Empty())))

and only in part 2:

assert(Empty().contains(0) == false)

Empty() should be replaced with End()

[davegurnell]

I forgot to reference this one in my recent PR but it is fixed now. Thanks!