readme: update discord link in invite URL
<p>
<a href="https://github.com/project-serum/anchor/actions"><img alt="Build Status" src="https://github.com/project-serum/anchor/actions/workflows/tests.yaml/badge.svg" /></a>
<a href="https://project-serum.github.io/anchor/"><img alt="Tutorials" src="https://img.shields.io/badge/docs-tutorials-blueviolet" /></a>
- <a href="https://discord.com/channels/889577356681945098"><img alt="Discord Chat" src="https://img.shields.io/discord/889577356681945098?color=blueviolet" /></a>
+ <a href="https://discord.gg/PDeRXyVURd"><img alt="Discord Chat" src="https://img.shields.io/discord/889577356681945098?color=blueviolet" /></a>
<a href="https://opensource.org/licenses/Apache-2.0"><img alt="License" src="https://img.shields.io/github/license/project-serum/anchor?color=blueviolet" /></a>
</p>
</div>
