fix(example): upgrades window customization example from deprecated cocoa to objc2-app-kit

[seanwatters]

Description

• What does this PR change? Give us a brief description.

Updates docs for changing window color on MacOS to use objc2-app-kit instead of deprecated cocoa crate.

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[FabianLars]

Thanks

[FabianLars]

Actually we have a background color setting now so we could remove the use of bindings here.