Update README.md with correct discord link (#6650)

Looks like the old Tari discord link was expired. Updated it with the
non-expiring link.

Description
---
Old Discord link was an expired link. Updated it with a new non-expiring
discord link.

Motivation and Context
---
Want to make sure that people reading the Tari README.md are able to get
correctly directed to the Tari Discord.

How Has This Been Tested?
---
The new, suggested Tari Discord link works.

What process can a PR reviewer use to test or verify this change?
---
Try the new, suggested Tari Discord link.

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Breaking Changes
---

- [x] None
- [ ] Requires data directory on base node to be deleted
- [ ] Requires hard fork
- [ ] Other - Please specify

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README.md

@@ -1017,4 +1017,4 @@ to generate the documentation. The generated html sits in `target/doc/`. Alterna
 
 ## Conversation channels
 
-We're generally on [Discord](https://discord.gg/q3Sfzb8S2V).
+We're generally on [Discord](https://discord.gg/tari).