[doc] Clear the timer on close (#1696)

In the heartbeat example, clear the timer on close.
websockets · Feb 16, 2020 · a39756a · a39756a
1 parent 9535702
commit a39756a
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diff --git a/README.md b/README.md
@@ -430,6 +430,10 @@ const interval = setInterval(function ping() {
     ws.ping(noop);
   });
 }, 30000);
+
+wss.on('close', function close() {
+  clearInterval(interval);
+});
 ```
 
 Pong messages are automatically sent in response to ping messages as required by