Resolve four broken refs in the docs
Hello!

## Pull Request overview
* Resolve 4 broken references in the docs

## The problems
Two broken references [here](https://huggingface.co/docs/datasets/package_reference/main_classes#datasets.Dataset.class_encode_column): 
![image](https://user-images.githubusercontent.com/37621491/220056232-366b64dc-33c9-461b-8f82-1ac4aa570280.png)

---

One broken reference [here](https://huggingface.co/docs/datasets/package_reference/main_classes#datasets.Dataset.unique): 
![image](https://user-images.githubusercontent.com/37621491/220057135-2f249d60-c01d-48b5-82bb-5085a7635198.png)

---

One missing reference [here](https://huggingface.co/docs/datasets/v2.9.0/en/package_reference/main_classes#datasets.DatasetDict.class_encode_column):
![image](https://user-images.githubusercontent.com/37621491/220057025-4a8e5556-5041-4ec7-b8d8-ed4fdc266495.png)

- Tom Aarsen