os/signal: don't expect to be notified for os.Kill

os.Kill cannot be caught on Unix systems.
The example gives the false impression that it can.

Fixes #13080.

Change-Id: I3b9e6f38a38f437a463c5b869ae84a0d3fd23f72
Reviewed-on: https://go-review.googlesource.com/16467Reviewed-by: Andrew Gerrand <adg@golang.org>

src/os/signal/example_test.go

@@ -15,7 +15,7 @@ func ExampleNotify() {
 	// We must use a buffered channel or risk missing the signal
 	// if we're not ready to receive when the signal is sent.
 	c := make(chan os.Signal, 1)
-	signal.Notify(c, os.Interrupt, os.Kill)
+	signal.Notify(c, os.Interrupt)
 
 	// Block until a signal is received.
 	s := <-c