Fix 'eval_split_name' described as defaulting to 'train'
# What does this PR do?

The default is correct (`test`) but the description is not.


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

Cc: @sgugger, @patil-suraj
_The documentation is not available anymore as the PR was closed or merged._