Fix typo with Replying message example

[prondubuisi]

I think there is a typo in the reply message example

$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']);

The third parameter for sendMessage should be $msg_bob['message']['id'] not $msg_june['message']['id'], as $msg_june is not declared anywhere within the snippet.

I am making a PR for this