Fixed broken link

Author: bkoelman

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,6 +4,6 @@ Closes #{ISSUE_NUMBER}
 
 #### QUALITY CHECKLIST
 - [ ] Changes implemented in code
-- [ ] Complies with our [contributing guidelines](./.github/CONTRIBUTING.md)
+- [ ] Complies with our [contributing guidelines](https://github.com/json-api-dotnet/JsonApiDotNetCore/blob/master/.github/CONTRIBUTING.md)
 - [ ] Adapted tests
 - [ ] Documentation updated