Update Topic.php

We can send message not only string, but array also, so, IDE does unpleasant highlights.

src/Ratchet/Wamp/Topic.php

@@ -38,7 +38,7 @@ class Topic implements \IteratorAggregate, \Countable {
 
     /**
      * Send a message to all the connections in this topic
-     * @param string $msg Payload to publish
+     * @param string|array $msg Payload to publish
      * @param array $exclude A list of session IDs the message should be excluded from (blacklist)
      * @param array $eligible A list of session Ids the message should be send to (whitelist)
      * @return Topic The same Topic object to chain