message: Tidy up PR template slightly
diff: - [ ] I have updated `CHANGELOG.next.toml` if I made changes to the smithy-rs codegen or runtime crates
  - [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS SDK, generated SDK code, or SDK runtime crates
  -By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.
  +----
  +
  +_By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice._
  