context: modernize AfterFunc example using WaitGroup.Go

The context.AfterFunc example currently uses the traditional
sync.WaitGroup pattern with Add and Done.

Update the example to use sync.WaitGroup.Go instead.

Fixes #78018

Change-Id: I079a773a6ec1c65f26af2bd8092067843adb1cd1
GitHub-Last-Rev: 9e648ae424
GitHub-Pull-Request: golang/go#78020
Reviewed-on: https://go-review.googlesource.com/c/go/+/752880
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/context/example_test.go

@@ -161,11 +161,8 @@ func ExampleAfterFunc_cond() {
 	cond := sync.NewCond(new(sync.Mutex))
 
 	var wg sync.WaitGroup
-	for i := 0; i < 4; i++ {
-		wg.Add(1)
-		go func() {
-			defer wg.Done()
-
+	for range 4 {
+		wg.Go(func() {
 			ctx, cancel := context.WithTimeout(context.Background(), 1*time.Millisecond)
 			defer cancel()
 
@@ -174,7 +171,7 @@ func ExampleAfterFunc_cond() {
 
 			err := waitOnCond(ctx, cond, func() bool { return false })
 			fmt.Println(err)
-		}()
+		})
 	}
 	wg.Wait()