Update slack channel url
</p>
<ul>
<li>
- User Forum: <a href="https://ei-slack.wso2.com/">Slack Channel</a>
+ User Forum: <a href="https://wso2-apim.slack.com/">Slack Channel</a>
</li>
<li>
User Forum: <a href="https://stackoverflow.com/questions/tagged/wso2mi">StackOverflow</a>
