Update GMMK Full ISO keymap to report correct layout

[reklrekl]

Description

Change 'KC_BSLS' to 'KC_NUHS' to report correct ISO keyboard layout

Types of Changes

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

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.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] 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).