[Docs] Fix example code in quantum_painter.md

[EricBanker12]

[Docs] Fix example code in quantum_painter.md

Fixes #24896

Description

Fixes example code that would unintentionally display an image 1 pixel up-left of the intended bottom-right corner position. Additionally, fixes edge case causing uint16_t overflow for images the same size as the display.

See #24896 for more details.

Types of Changes

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

Issues Fixed or Closed by This PR

• [Docs] Quantum Painter Draw Image Example Code Is Wrong And Causes uint16_t Overflow #24896

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).