Updater gitter.im badge with correct link.

README.md

 [![Build Status](https://travis-ci.org/magento/magento2.svg?branch=master)](https://travis-ci.org/magento/magento2)
-[![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/composer/composer?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
+[![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/magento/magento2?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
 <h2>Welcome</h2>
 Welcome to Magento 2 installation! We're glad you chose to install Magento 2, a cutting edge, feature-rich eCommerce solution that gets results.