windows: pass rune to %q formatting verb

It's caught by Go 1.26's vet check that the format %q here
is given a wrong type uint16. Convert it to a rune instead.

Change-Id: Ic6eb4d0c64c898f008ca47e694ac2f45d2cd9965
Reviewed-on: https://go-review.googlesource.com/c/sys/+/818181
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ayman Bagabas <ayman.bagabas@gmail.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>