Also update another reference to the developer documentation

Author: samwierema

README.md

@@ -1,6 +1,6 @@
 MessageBird's REST API for PHP
 ===============================
-This repository contains the open source PHP client for MessageBird's REST API. Documentation can be found at: https://www.messagebird.com/developers/php
+This repository contains the open source PHP client for MessageBird's REST API. Documentation can be found at: https://developers.messagebird.com/
 
 [![Build Status](https://travis-ci.org/messagebird/php-rest-api.svg?branch=master)](https://travis-ci.org/messagebird/php-rest-api)
 [![Latest Stable Version](https://poser.pugx.org/messagebird/php-rest-api/v/stable.svg)](https://packagist.org/packages/messagebird/php-rest-api)