chore(github): update pr template

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,8 +1,14 @@
-*Before* submitting a pull request, please make sure the following is done...
+_Before_ submitting a pull request, please make sure the following is done...
 
 - [ ] Ensure the pull request title and commit message follow the [Commit Specific](https://github.com/alibaba/formily/blob/formily_next/.github/GIT_COMMIT_SPECIFIC.md) in **English**.
 - [ ] Fork the repo and create your branch from `master` or `formily_next`.
 - [ ] If you've added code that should be tested, add tests!
 - [ ] If you've changed APIs, update the documentation.
 - [ ] Ensure the test suite passes (`npm test`).
 - [ ] Make sure your code lints (`npm run lint`) - we've done our best to make sure these rules match our internal linting guidelines.
+
+**Please do not delete the above content**
+
+---
+
+## What have you changed?