Add an example of Redux Toolkit and `@typescript-eslint/no-floating-promises`

Following https://github.com/reduxjs/redux-toolkit/issues/4101 -> https://github.com/reduxjs/redux-toolkit/pull/4102, I don't know that there's a single place online showing using [`allowForKnownSafePromises`](https://typescript-eslint.io/rules/no-floating-promises/#allowforknownsafepromises).