errors: fix package example

The example in example_test.go requires that the whole file be
displayed; the addition of ExampleAs meant that only the body of the
package example function was shown, rather than the surrounding context.

This change moves ExampleAs to the file wrap_test.go file, restoring the
package example to its former glory.

Update #31716

Change-Id: Id0ea77bc06023b239a63c1d6a7c8b3c1dae91ce9
Reviewed-on: https://go-review.googlesource.com/c/go/+/179737


Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Jean de Klerk <deklerk@google.com>
Run-TryBot: Benny Siegert <bsiegert@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>