docs: Use valid plugin name in ESLint v8 instructions

[kamilogorek]

Removed incorrecly used @.

[bruce-c-liu]

@veritem

I don't think this fix is correct. Things were working with #499, but this breaks it again.

[bruce-c-liu]

Oh, I see. It's because @kamilogorek was using the "deprecated" eslint-plugin-vitest package instead of the now "official" @vitest/eslint-plugin package.

Related issue: #537

@kamilogorek Please revert this change and use @vitest/eslint-plugin instead. Thanks :)

[kamilogorek]

Ah gotcha, can confirm everything works just fine with @vitest/eslint-plugin. Cheers!