docs: fix broken links in README and CONTRIBUTING (ArroyoSystems#1018)

- README.md: fix malformed tutorial link (had extra '[' and mismatched parens)
  '[tutorial]([https://...])'  →  '[tutorial](https://...)'
- CONTRIBUTING.md: fix Discord link that incorrectly pointed to dev-setup page
  instead of the actual Discord server invite URL

Author: cluster2600

CONTRIBUTING.md

@@ -3,5 +3,5 @@
 We welcome contributions from the community!
 
 Please refer to the [dev setup](https://doc.arroyo.dev/developing/dev-setup) guide for how to get started. You can
-find help on our [discord](https://doc.arroyo.dev/developing/dev-setup) or via email at
+find help on our [discord](https://discord.gg/cjCr5rVmyR) or via email at
 [support@arroyo.systems](mailto:support@arroyo.systems).

README.md

@@ -105,7 +105,7 @@ Then, load the Web UI at http://localhost:5115.
 
 For a more in-depth guide, see the [getting started guide](https://doc.arroyo.dev/getting-started).
 
-Once you have Arroyo running, follow the [tutorial]([https://doc.arroyo.dev/tutorial/first-pipeline/)) to create your first real-time
+Once you have Arroyo running, follow the [tutorial](https://doc.arroyo.dev/tutorial/first-pipeline/) to create your first real-time
 pipeline.