docs: os.cpu() should be os.cpus()

[KermanX]

Description

os.cpu() should be os.cpus() in config docs.

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[AriPerkkio]

Same change is needed here on JSDoc:

vitest/packages/vitest/src/types/coverage.ts

Lines 209 to 213 in ca87027

	/**
	* Concurrency limit used when processing the coverage results.
	* Defaults to `Math.min(20, os.cpu().length)`
	*/
	processingConcurrency?: number