Update pull_request_template.md

Fixed broken link to CONTRIBUTING Guidelines

Author: KarolinHem

.github/pull_request_template.md

@@ -13,4 +13,4 @@
 - [ ] I have kept the patch limited to only change the parts related to the patch
 - [ ] This change requires a documentation update
 
-See also [Contributing Guidelines](CONTRIBUTING.md).
+See also [Contributing Guidelines](/CONTRIBUTING.md).