os: fix PathError.Op for dirFS.Open

os: fix PathError.Op for dirFS.Open

This appears to be a copy-paste error from CL 455362.

The operation name used to be "open"
but seems to have been accidentally changed to "stat".
This CL reverts back to "open".

Change-Id: I3fc5168095e2d9eee3efa3cc091b10bcf4e3ecde
Reviewed-on: https://go-review.googlesource.com/c/go/+/539056
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>