[Documentation] Minor change: Unify indexes in Variance of generic types description

[hnykda]

This is a very minor fix.

The last comment in this code block:

my_circles: list[Circle] = []
add_one(my_circles)     # This may appear safe, but...
my_circles[-1].rotate()  # ...this will fail, since my_circles[0] is now a Shape, not a Circle

uses a different index 0 instead of the one used in the code -1. I think these should be same to avoid confusion. I went for 0.

[hauntsaninja]

Thanks!