Update tickers.go to not leak goroutine

The code in the example will leak a goroutine because ticker.Stop() does not close ticker.C.
There is no way to close ticker.C, so `for _ := range ticker` is not a generally safe example for using a ticker.

examples/tickers/tickers.go

@@ -16,10 +16,17 @@ func main() {
     // `range` builtin on the channel to iterate over
     // the values as they arrive every 500ms.
     ticker := time.NewTicker(500 * time.Millisecond)
+    done := make(chan bool)
+
     go func() {
-        for t := range ticker.C {
+        for {
+            select {
+            case <-done:
+                return
+            case t := <-ticker.C:
                 fmt.Println("Tick at", t)
             }
+        }
     }()
 
     // Tickers can be stopped like timers. Once a ticker
@@ -27,5 +34,6 @@ func main() {
     // channel. We'll stop ours after 1600ms.
     time.Sleep(1600 * time.Millisecond)
     ticker.Stop()
+    done <- true
     fmt.Println("Ticker stopped")
 }