math/big: fix comment in divRecursiveStep

There appears to be a typo in the description of
the recursive division algorithm.

Two things seem suspicious with the original comment:
  1. It is talking about choosing s, but s doesn't
     appear anywhere in the equation.
  2. The math in the equation is incorrect.

Where
  B = len(v)/2
  s = B - 1

Proof that it is incorrect:
    len(v) - B >= B + 1
    len(v) - len(v)/2 >= len(v)/2 + 1

    This doesn't hold if len(v) is even, e.g. 10:
    10 - 10/2 >= 10/2 + 1
    10 - 5 >= 5 + 1
    5 >= 6  // this is false

The new equation will be the following,
which will be mathematically correct:
    len(v) - s >= B + 1
    len(v) - (len(v)/2 - 1) >= len(v)/2 + 1
    len(v) - len(v)/2 + 1 >= len(v)/2 + 1
    len(v) - len(v)/2 >= len(v)/2

    This holds if len(v) is even or odd.

    e.g. 10
    10 - 10/2 >= 10/2
    10 - 5 >= 5
    5 >= 5

    e.g. 11
    11 - 11/2 >= 11/2
    11 - 5 >= 5
    6 >= 5

Change-Id: If77ce09286cf7038637b5dfd0fb7d4f828023f56
Reviewed-on: https://go-review.googlesource.com/c/go/+/287372


Run-TryBot: Katie Hockman <katie@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Trust: Katie Hockman <katie@golang.org>