Update compatible_microcontrollers.md

[ywngr]

Description

fix filename extension for platformdev_rp2040 page link

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

Checklist

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

[zvecr]

#24151