Update README.md

Author: haphan

README.md

@@ -23,7 +23,7 @@ Due to new [object typehint](https://wiki.php.net/rfc/object-typehint) since PHP
 
 ## Getting help
 
-- Meet us on Slack: https://phpopencloud.slack.com ([Get your invitation](https://slackpass.io/phpopencloud))
+- Meet us on Slack: https://phpopencloud.slack.com ([Get your invitation](https://launchpass.com/phpopencloud))
 - Report and issue: https://github.com/php-opencloud/openstack/issues
 
 ## Requirements