Update PULL_REQUEST_TEMPLATE.md

fixing the broken contributing guide link

.github/PULL_REQUEST_TEMPLATE.md

 <!-- Thank you for taking the time to work on a Pull Request for this project! -->
 <!-- To ensure your PR is dealt with swiftly please check the following: -->
-- [ ] My submission is formatted according to the guidelines in the [contributing guide](CONTRIBUTING.md)
+- [ ] My submission is formatted according to the guidelines in the [contributing guide](/CONTRIBUTING.md)
 - [ ] My addition is ordered alphabetically
 - [ ] My submission has a useful description
 - [ ] The description does not end with punctuation