Fix URL to contributing guide (#3141)

Author: lukasbindreiter

.github/CONTRIBUTING.md

@@ -1,4 +1,4 @@
 Contributing
 ============
 
-Please see the [project documentation](https://zarr.readthedocs.io/en/stable/contributing.html) for information about contributing to Zarr.
+Please see the [project documentation](https://zarr.readthedocs.io/en/stable/developers/contributing.html) for information about contributing to Zarr.