ios/fs: mention f.dir in (*subFS).fixErr godoc

There is no dir parameter to (f *subFS).fixErr.

Change-Id: I49e42bac5e102cfab0d289658d9871429cfec515
Reviewed-on: https://go-review.googlesource.com/c/go/+/292389
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/io/fs/sub.go

@@ -68,7 +68,7 @@ func (f *subFS) shorten(name string) (rel string, ok bool) {
 	return "", false
 }
 
-// fixErr shortens any reported names in PathErrors by stripping dir.
+// fixErr shortens any reported names in PathErrors by stripping f.dir.
 func (f *subFS) fixErr(err error) error {
 	if e, ok := err.(*PathError); ok {
 		if short, ok := f.shorten(e.Path); ok {