Update pr_checklist.md

[Jels02]

Update readme.md template link from https://github.com/qmk/qmk_firmware/blob/master/data/templates/avr/readme.md to https://github.com/qmk/qmk_firmware/blob/master/data/templates/keyboard/readme.md

Description

There was a broken link in the PR checklist, this change fixes that link. Check commit for details.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

• See above

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).