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 <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>

src/io/fs/example_test.go

@@ -0,0 +1,25 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs_test
+
+import (
+	"fmt"
+	"io/fs"
+	"log"
+	"os"
+)
+
+func ExampleWalkDir() {
+	root := "/usr/local/go/bin"
+	fileSystem := os.DirFS(root)
+
+	fs.WalkDir(fileSystem, ".", func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			log.Fatal(err)
+		}
+		fmt.Println(path)
+		return nil
+	})
+}