Fix broken link to contribution guidelines in pull request template

Author: bouweandela

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 **Description**
 
-- [ ] I have read the [contribution guidelines](../CONTRIBUTING.md)
+- [ ] I have read the [contribution guidelines](https://github.com/NLeSC/python-template/blob/main/CONTRIBUTING.md)
 - [ ] This update is in line with what is recommended in the [Python chapter of the Guide](https://guide.esciencecenter.nl/#/best_practices/language_guides/python)
 - [ ] All user facing changes have been added to CHANGELOG.md