Update rgb_matrix.md

[lizaoreo]

Description

Changed the example in indicator-examples-2 to use a compound literal, otherwise the code fails to compile.

Types of Changes

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

Issues Fixed or Closed by This PR

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.
• [x] 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).