update discord link

pages/blog/pulls.md

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   together and track them across multiple commits, and to
   provide "history" for each change. This works great with [Jujutsu][jj].
 
-Join us on [Discord](https://discord.gg/W7SRR4xHhd) or
+Join us on [Discord](https://chat.tangled.sh) or
 `#tangled` on libera.chat (the two are bridged, so we will
 never miss a message!). We are always available to help
 setup knots, listen to feedback on features, or even