Update CONTRIBUTING.md

Fix relative link in contributing.md

When viewed on GitHub's web interface, the relative link was interpreted as relative to the current file path, leading to the incorrect URL "https://github.com/Rust-GPU/rust-gpu/blob/main/discussions" instead of the intended Discussions tab.

Author: doomdagadiggiedahdah

CONTRIBUTING.md

@@ -5,9 +5,7 @@ Welcome! We're glad you're interested in contributing! We welcome contributions
 ## How to get help
 
 For questions, clarifications, and general help please first search [Github
-discussions](discussions) and then ask your question if you can't find the answer.
-
-[discussions]: https://github.com/rust-gpu/rust-gpu/discussions
+discussions](https://github.com/rust-gpu/rust-gpu/discussions) and then ask your question if you can't find the answer.
 
 ## Issues