hash/maphash: add package-level example
hash/maphash: add package-level example

Change-Id: I05c7ca644410822a527e94a7a8b883a0f8b0a4ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/220420
Run-TryBot: Alberto Donizetti <90b16103a35511ced020676f3544f17117fb4c92@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
Reviewed-by: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>

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