sync: improve WaitGroup example by putting the call to Done in a

deferred block. This makes hangs in the waiting code less likely
if a goroutine exits abnormally.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/7306052

src/pkg/sync/example_test.go

@@ -24,10 +24,10 @@ func ExampleWaitGroup() {
 		wg.Add(1)
 		// Launch a goroutine to fetch the URL.
 		go func(url string) {
+			// Decrement the counter when the goroutine completes.
+			defer wg.Done()
 			// Fetch the URL.
 			http.Get(url)
-			// Decrement the counter.
-			wg.Done()
 		}(url)
 	}
 	// Wait for all HTTP fetches to complete.
@@ -37,7 +37,7 @@ func ExampleWaitGroup() {
 func ExampleOnce() {
 	var once sync.Once
 	onceBody := func() {
-		fmt.Printf("Only once\n")
+		fmt.Println("Only once")
 	}
 	done := make(chan bool)
 	for i := 0; i < 10; i++ {