[Keyboard] Fix default keymap for GMMK Pro Iso

[drashna]

Description

Fix encoder define in default keymap

Types of Changes

• Bugfix
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)

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).

[andrebrait]

Oops 😅

[drashna]

No worries!