Clean up the PR template for improved readability

[karlhigley]

Description

Clean up the PR template for improved usability

Checklist

• [ ] I did follow the Contribution Guidelines
• [X] I did follow the Code of Conduct
• [ ] I have commented my code following the OpenMined Styleguide
• [ ] I have added tests for my changes

[cereallarceny]

I'm good with this @karlhigley, merging.