Fix deprecated warning message and docstring
I have a question: what about `dictionary_encode_column_`?
- It is deprecated in Dataset, but it recommends using a non-existing method instead: `Dataset.dictionary_encode_column` does not exist.
- It is NOT deprecated in DatasetDict. `dictionary_encode_column_ ` should be deprecated since it never worked correctly. It will be removed in a major release.
This has to be deprecated in `DatasetDict` as well.
And `Dataset.dictionary_encode_column` doesn't exist indeed. Thanks @lhoestq. I have fixed deprecated for `dictionary_encode_column_`.
Fix deprecated warnings:
- Use deprecated Sphinx directive in docstring
- Fix format of deprecated message
- Raise FutureWarning