log/slog: make DiscardHandler example package-level

Fixes #70782.

Change-Id: I8e8b763040bd10147eb7d1a30ac0774e28f90911
Reviewed-on: https://go-review.googlesource.com/c/go/+/635217
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/log/slog/example_discard_test.go

@@ -10,7 +10,7 @@ import (
 	"os"
 )
 
-func ExampleDiscardHandler() {
+func Example_discardHandler() {
 	// A slog.TextHandler can output log messages.
 	logger1 := slog.New(slog.NewTextHandler(
 		os.Stdout,