Fix contributing link in PR template

Author: asgrim

.github/pull_request_template.md

@@ -7,7 +7,7 @@
 
 ## PR submitter checklist
 
-- [ ] I have read [CONTRIBUTING.md](../CONTRIBUTING.md)
+- [ ] I have read [CONTRIBUTING.md](https://github.com/php/pie/blob/HEAD/CONTRIBUTING.md)
 - [ ] I discussed this <bug|feature> with the maintainers in #<issue_number> (complete as appropriate)
 - [ ] I have added appropriate tests
 - [ ] I confirm that I have the right to submit this under the project's open source licence