gh-35141: Revise the PR template
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### :books: Description
Made small edits to the PR template, making it concise and tidy overall.
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### :memo: Checklist
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- [x] The title is concise, informative, and self-explanatory.
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- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/35141
Reported by: Kwankyu Lee
Reviewer(s): Kwankyu Lee, Tobias Diez
