One more slash for variable output_prefix

In the notebook of `community/codelabs/docai-ocr/batch_processing.py`

When I deal with more than 10 docs, it will produce duplicated results for the next step. After checking, I found the variable `output_prefix` need one more slash `/` to recognize the paths of generated jsons.

I have seen other tutorials also missed this problem like [https://cloud.google.com/document-ai/docs/send-request#documentai_batch_process_document-python](url) and [https://www.cloudskillsboost.google/course_templates/686/labs/455608](url)

Thank you for your attention.