math/big: fix typo in documentation for Int.Exp

Fixes #38304

Also change `If m > 0, y < 0, ...` to `If m != 0, y < 0, ...` since `Exp` will return `nil`
whatever `m`'s sign is.

Change-Id: I17d7337ccd1404318cea5d42a8de904ad185fd00
GitHub-Last-Rev: 23995103000505dbf35aa29a717470c4da638fda
GitHub-Pull-Request: golang/go#38390
Reviewed-on: https://go-review.googlesource.com/c/go/+/228000


Reviewed-by: Robert Griesemer <gri@golang.org>