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

Link to failing job: https://github.com/huggingface/transformers/actions/runs/4867713745/jobs/8680544136

This PR fixes the current failing doctest for pix2struct. https://github.com/huggingface/transformers/pull/23051 fixed the issues related with pix2struct and training by changing the way attention masks are computed. Naturally this has changed the value of the expected loss function in the docstring

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