docs: Fix documentation default value for the --pool option

[austinhnelson]

Description

Fixes #7035
The CLI docs state the wrong default value for the --pool option. It was changed from threads to forks in 2.0:
https://vitest.dev/guide/migration.html
https://vitest.dev/guide/cli.html#pool

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[AriPerkkio]

Looks good, thanks!