io/fs: fix reference to WalkFunc

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>