errors: add example for Is
errors: add example for Is

Add ExampleIs to illustrate how errors.Is works.

Updates #31716.
Updates #38369.

Change-Id: I1b9a6667614635aa3a5ed8b2c108d8eb6f35748b
Reviewed-on: https://go-review.googlesource.com/c/go/+/228038
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

CAFxX/go,nawawi/go,akutz/go,akutz/go,akutz/go,nawawi/go,cloudflare/go,golang/go,golang/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,golang/go,akutz/go,nawawi/go,cloudflare/go,akutz/go,cloudflare/go,akutz/go,akutz/go,cloudflare/go,akutz/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,nawawi/go