macOS: Fix overridden fullscreen selectors

[madsmtm]

Fixes #2520

These were incorrectly named (Fullscreen instead of FullScreen, notice the big "S").

• Tested on all platforms changed
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
  • Introduced in Clean up macOS class declaration #2458 which has not been released yet, so no need to note this in the changelog.
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

[rofferom]

Works on my machine 👍

[madsmtm]

For reference: I'm working on things in objc2 that could help catching mistakes like these in the future