[`Pix2Struct`] Fix pix2struct doctest
# What does this PR do?

Fixes Pix2Struct doctest
Link to failing job: https://github.com/huggingface/transformers/actions/runs/4815336726/jobs/8573921590

With https://github.com/huggingface/transformers/pull/23004 being merged, the label smoothing of the loss function has been removed. Therefore the expected value of the loss function changed, leading to the failing doctest.

cc @ydshieh 

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