Typo in Sequence-to-sequence models unit title

[sergiopaniego]

In unit Sequence-to-sequence models, 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]]