Fix undefined variable in reply message example (#96)

Author: prondubuisi

README.md

@@ -98,7 +98,7 @@ $channel->addMembers(['mike', 'joe']);
 $msg_bob = $channel->sendMessage(["text" => "Hi June!"], 'bob-1');
 
 // Reply to a message
-$reply_bob = $channel->sendMessage(["text" => "Long time no see!"], 'bob-1', $msg_june['message']['id']);
+$reply_bob = $channel->sendMessage(["text" => "Long time no see!"], 'bob-1', $msg_bob['message']['id']);
 ```
 
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