docs: Resolve incorrect type typo in trainer methods
# What does this PR do?
Replace incorrect `Lst[str]` with `List[str]` in docstrings in various locations in the `Trainer`.

## Before submitting
- [x] This PR fixes a typo or improves the docs

## Who can review?

Documentation: @sgugger

* Tom Aarsen
_The documentation is not available anymore as the PR was closed or merged._