Fix reflinks in PR template
## Checklist
-- [ ] The PR conforms to DataHub's [Contributing Guideline](https://github.com/linkedin/datahub/blob/master/CONTRIBUTING.md) (particularly [Commit Message Format](https://github.com/linkedin/datahub/blob/master/CONTRIBUTING.md#commit-message-format))
+- [ ] The PR conforms to DataHub's [Contributing Guideline](https://github.com/linkedin/datahub/blob/master/docs/CONTRIBUTING.md) (particularly [Commit Message Format](https://github.com/linkedin/datahub/blob/master/docs/CONTRIBUTING.md#commit-message-format))
- [ ] Links to related issues (if applicable)
- [ ] Tests for the changes have been added/updated (if applicable)
- [ ] Docs related to the changes have been added/updated (if applicable)
