[Translation] Fix fallback parameter name in the framework config

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | -
| Fixed tickets | -

There is no `fallback_locale` parameter in the `framework` configuration, it is called `fallback` under the `translator`config parameter. Should it be indicated somehow that `fallback` is located under `translator` like `translator.fallback`? I couldn't find any similar approach though.