update docs according to new version of intervalPlural-postProcessor

[vitek-rostislav]

• see https://github.com/i18next/i18next-intervalPlural-postProcessor/commit/68c6f90b2cca9492299d618c1678e6ef170080b6

Checklist (for documentation change)

• [x] only relevant documentation part is changed (make a diff before you submit the PR)
• [x] motivation/reason is provided

[jamuhl]

right...completely forgot we mentioned that in the docs...thank you for updating it 👍