Pin PEFT for the languge-modeling example

[regisss]

What does this PR do?

Temporary fix for #590 while we determine the reason for higher memory consumption with PEFT v0.7.0.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.