[docs] Fix TF base model examples: outputs.last_hidden_states -> state
# What does this PR do?

Fixes a typo in the examples of Tensorflow-based base models, in which the returned last_hidden_state attribute of model output is incorrectly listed as "last_hidden_states".

Fixes #9376 

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

## Who can review?

@julien-c @patrickvonplaten
