Typo in Sequence-to-sequence models unit title

In unit [Sequence-to-sequence models](https://huggingface.co/learn/nlp-course/chapter1/7?fw=pt), it seems like there is a typo in the title that should be a reference:

Instead, I think that it should be just **Sequence-to-sequence models** as in other unit titles:

```
# Decoder models[[decoder-models]]
# Sequence-to-sequence models[[sequence-to-sequence-models]]
```