Update PULL_REQUEST_TEMPLATE.md

Author: lowlydba

.github/PULL_REQUEST_TEMPLATE.md

@@ -32,6 +32,6 @@
 - [ ] My code follows the code style of this project.
 - [ ] My change requires a change to the documentation.
 - [ ] I have updated the documentation accordingly.
-- [ ] I have read the **CONTRIBUTING** document.
+- [ ] I have read the [**CONTRIBUTING**](../CONTRIBUTING.md) document.
 - [ ] I have added tests to cover my changes.
 - [ ] All new and existing tests passed.