os/signal: windows is supported, update comment

Fixes #5035.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7519045

src/pkg/os/signal/signal.go

@@ -5,7 +5,7 @@
 // Package signal implements access to incoming signals.
 package signal
 
-// BUG(rsc): This package is not yet implemented on Plan 9 and Windows.
+// BUG(rsc): This package is not yet implemented on Plan 9.
 
 import (
 	"os"