io/fs: fix reference to WalkFunc

The comment explains differences between WalkDirFunc and WalkFunc,
but when this code moved out of path/filepath, we forgot to change
the reference to be filepath.WalkFunc. Fix that.

(The text should not be deleted, because path/filepath does not
contain this type - WalkDirFunc - nor this text anymore.)

Pointed out by Carl Johnson on CL 243916 post-submit.

For #41190.

Change-Id: I44c64d0b7e60cd6d3694cfd6d0b95468ec4612fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/268417
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/src/io/fs/walk.go b/src/io/fs/walk.go
index e50c1bb..dfc7376 100644
--- a/src/io/fs/walk.go
+++ b/src/io/fs/walk.go
@@ -9,7 +9,7 @@
 	"path"
 )
 
-// SkipDir is used as a return value from WalkFuncs to indicate that
+// SkipDir is used as a return value from WalkDirFuncs to indicate that
 // the directory named in the call is to be skipped. It is not returned
 // as an error by any function.
 var SkipDir = errors.New("skip this directory")
@@ -56,7 +56,7 @@
 // after a failed ReadDir and reports the error from ReadDir.
 // (If ReadDir succeeds, there is no second call.)
 //
-// The differences between WalkDirFunc compared to WalkFunc are:
+// The differences between WalkDirFunc compared to filepath.WalkFunc are:
 //
 //   - The second argument has type fs.DirEntry instead of fs.FileInfo.
 //   - The function is called before reading a directory, to allow SkipDir