[Docs] `prefer-stateless-function`: fix docs for `ignorePureComponents` option.
[Docs] `prefer-stateless-function`: fix docs for `ignorePureComponents` option.

Per https://github.com/yannickcr/eslint-plugin-react/pull/781/files#r76556839
petersendidit/eslint-plugin-react,yannickcr/eslint-plugin-react