[release-branch.go1.13] errors: fix wrong code in package doc
You can't call Unwrap on the return value of fmt.Errorf, but
you can pass the result to errors.Unwrap.
Also, move the description of the Unwrap function up so the
example makes sense.
Fixes #34082.
Change-Id: Ica07c44665c5e65deea4aa6a146fc543a5a0a99d
Reviewed-on: https://go-review.googlesource.com/c/go/+/193298
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Katie Hockman <katie@golang.org>
(cherry picked from commit
d9a3d902ec139c95d8dc1b69977783fb8134b552)
Reviewed-on: https://go-review.googlesource.com/c/go/+/193263
TryBot-Result: Gobot Gobot <gobot@golang.org>