Fix function reference in "types-variance" chapter

[matetukacs]

The last paragraph of the "types-variance" chapter explains how type variance applies to Scala functions.

In the last of the 3 examples that "illustrate the subtyping relationships induced by variance annotations on functions", there is a typo:
val h: Function[Book, Buyable] = g

g cannot be assigned to h, as B is not (and cannot) be contravariant.

I believe the intention was to assign g to f, to illustrate the contravariance of A.

Alternatively, the type of h can be modified to Function[Book, Item] to allow the assignment.

[julienrf]

Good catch! 🤓

[matetukacs]

Good catch! 🤓

thank you! 🙇