[doc] fix: fix slack invitation link (verl-project#3230)

Author: eric-haibin-lin

README.md

@@ -10,7 +10,7 @@
 <a href="https://deepwiki.com/volcengine/verl"><img src="https://devin.ai/assets/deepwiki-badge.png" alt="Ask DeepWiki.com" style="height:20px;"></a>
 [![GitHub Repo stars](https://img.shields.io/github/stars/volcengine/verl)](https://github.com/volcengine/verl/stargazers)
 [![Twitter](https://img.shields.io/twitter/follow/verl_project)](https://twitter.com/verl_project)
-<a href="https://join.slack.com/t/verlgroup/shared_invite/zt-2w5p9o4c3-yy0x2Q56s_VlGLsJ93A6vA"><img src="https://img.shields.io/badge/Slack-verl-blueviolet?logo=slack&amp"></a>
+<a href="https://join.slack.com/t/verl-project/shared_invite/zt-3c6mc2khw-v0lo6NfDPuFP6OnkrZwfqw"><img src="https://img.shields.io/badge/Slack-verl-blueviolet?logo=slack&amp"></a>
 <a href="https://arxiv.org/pdf/2409.19256"><img src="https://img.shields.io/static/v1?label=EuroSys&message=Paper&color=red"></a>
 [![Documentation](https://img.shields.io/badge/documentation-blue)](https://verl.readthedocs.io/en/latest/)
 <a href="https://raw.githubusercontent.com/eric-haibin-lin/verl-community/refs/heads/main/WeChat.JPG"><img src="https://img.shields.io/badge/微信-green?logo=wechat&amp"></a>