docs: Change discord url to an invite link (#235)

docs/src/getting-started/introduction.md

@@ -13,5 +13,5 @@ Here, we'll walkthrough several tutorials demonstrating how to use Anchor. To sk
 [quickstart](./quick-start.md) or go [here](https://github.com/project-serum/anchor/blob/master/examples/tutorial/basic-2/programs/basic-2/src/lib.rs). For an introduction to Solana, see the [docs](https://docs.solana.com/developing/programming-model/overview).
 
 ::: tip NOTE
-Anchor is in active development, so all APIs are subject to change. If you are one of the early developers to try it out and have feedback, please reach out by [filing an issue](https://github.com/project-serum/anchor/issues/new). This documentation is a work in progress and is expected to change dramatically as features continue to be built out. If you have any problems, consult the [source](https://github.com/project-serum/anchor) or feel free to ask questions on the [Serum Discord](https://discord.com/channels/739225212658122886/752530209848295555).
+Anchor is in active development, so all APIs are subject to change. If you are one of the early developers to try it out and have feedback, please reach out by [filing an issue](https://github.com/project-serum/anchor/issues/new). This documentation is a work in progress and is expected to change dramatically as features continue to be built out. If you have any problems, consult the [source](https://github.com/project-serum/anchor) or feel free to ask questions on the [Serum Discord](https://discord.gg/rg5ZZPmmTm).
 :::

docs/src/tutorials/tutorial-3.md

@@ -73,4 +73,4 @@ retrieve the return value. In future work, Anchor should do this transparently.
 
 That's it for the introductory tutorials for Anchor. For more, see the `examples/` directory in the
 [repository](https://github.com/project-serum/anchor). For feature requests or bugs, GitHub issues
-are appreciated. For direct questions or support, join the [Serum Discord](https://discord.com/channels/739225212658122886/752530209848295555).
+are appreciated. For direct questions or support, join the [Serum Discord](https://discord.gg/rg5ZZPmmTm).