Update test_image_processing_pix2struct.py
# What does this PR do?

This PR should fix the failing test on `main`. 
The fix is to replace the previous image with the one I have uploaded on the Hub: https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/transformers/tasks/australia.jpg

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