clarify AI policy checkbox

Updated the pull request template to clarify issue closing and AI usage.

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,7 +2,10 @@
 Thanks for your contribution!
 -->
 
-- [ ] Closes #xxxx <!-- Replace xxxx with the GitHub issue number -->
+- [ ] Closes #xxxx <!-- Replace xxxx with the GitHub issue number if exists -->
+
+One of checkbox below must be checked.
+- [ ] I did not use AI to write the code of this patch.
 - [ ] This PR follows our [AI policy](https://github.com/RustPython/.github/blob/main/AI_POLICY.md)
 
 ## Summary