testing/fstest: fix typo in error message

Change-Id: Iac59f5271c79c46b39733fdf0eb4bf9b0fc0bdca
GitHub-Last-Rev: 03f96e32a81d1516a9307b6578c930434783e3d3
GitHub-Pull-Request: golang/go#43450
Reviewed-on: https://go-review.googlesource.com/c/go/+/280953
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/testing/fstest/testfs.go

@@ -121,7 +121,7 @@ func (t *fsTester) openDir(dir string) fs.ReadDirFile {
 	d, ok := f.(fs.ReadDirFile)
 	if !ok {
 		f.Close()
-		t.errorf("%s: Open returned File type %T, not a io.ReadDirFile", dir, f)
+		t.errorf("%s: Open returned File type %T, not a fs.ReadDirFile", dir, f)
 		return nil
 	}
 	return d