fix invalid CONTRIBUTING link in PR template

Author: fmigneault

.github/PULL_REQUEST_TEMPLATE.md

@@ -21,7 +21,7 @@
 
 <!-- Mark with an `x` all the checkboxes that apply (like `[x]`) -->
 
-- [ ] I've read the [`CONTRIBUTING.md`](../CONTRIBUTING.md) guide;
+- [ ] I've read the [`CONTRIBUTING.md`](https://github.com/stac-extensions/mlm/blob/main/CONTRIBUTING.md) guide;
 - [ ] I've updated the code style using `make check`;
 - [ ] I've written tests for all new methods and classes that I created;
 - [ ] I've written the docstring in `Google` format for all the methods and classes that I used.