Fix a doc and update an example

examples/subscribe-and-unsubscribe.php

@@ -3,14 +3,25 @@
 use m50\Pusher\Client;
 use m50\Pusher\Event;
 
+use function Amp\async;
+use function Amp\delay;
+
 include_once('vendor/autoload.php');
 
 $client = Client::create('my-app', 'us3');
 
 $channel = $client->channel('my-channel');
-$channel->subscribe(function (Event $event) use ($channel) {
-    var_dump($event);
-    if ($event->event === 'END') {
-        $channel->unsubscribe();
-    }
+async(function () use ($channel) {
+    echo 'in async' . PHP_EOL;
+    $channel->subscribe(function (Event $event) use ($channel) {
+        var_dump($event);
+        if ($event->event === 'END') {
+            $channel->unsubscribe();
+        }
+    });
 });
+
+delay(30);
+echo 'delayed' . PHP_EOL;
+$client->close();
+echo 'closed' . PHP_EOL;

src/Channel.php

@@ -25,7 +25,8 @@ public function unsubscribe()
     /**
      * Execute an action on every event that belongs to the channel.
      *
-     * @param (callable(Event $event): void) $onEvent
+     * @param (callable(Event): void) $onEvent
+     * @param (callable(\Throwable): void) $onError
      * @return void
      */
     public function subscribe(callable $onEvent, ?callable $onError = null): void