docs: update PR template
## Checklist
- [ ] The PR conforms to DataHub's [Contributing Guideline](https://github.com/linkedin/datahub/blob/master/CONTRIBUTING.md)
+- [ ] Links to related issues (if applicable)
- [ ] Tests for the changes have been added/pudated (if applicable)
- [ ] Docs related to the changes have been added/updated (if applicable)
