io/fs: added an example for io/fs.WalkDir
io/fs: added an example for io/fs.WalkDir

The documentation currently does not show how to get an `FS` instance for the operating system's filesystem. This example demonstrates how to accomplish this using the `os` package.

Fixes #46083

Change-Id: I053111c12ab09ef13f0d04fcdff8a6ea0dccf379
Reviewed-on: https://go-review.googlesource.com/c/go/+/319989
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Trust: Emmanuel Odeke <bd234ba4276433f0e5fc7a8fa2d18274fa711567@orijtech.com>

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