Update default revision for document-question-answering

[ankrgyl]

What does this PR do?

Prior to this change, users needed to instantiate a tokenizer themselves while using impira/layoutlm-document-qa to set the add_prefix_space=True parameter. I made this the default in the tokenizer's config here, and this change simply updates the pinned revision to reference it.

After this change, the following commands work:

In [1]: from transformers import AutoTokenizer, pipeline
In [2]: nlp = pipeline('document-question-answering')
In [3]: nlp(
   ...:     "https://templates.invoicehome.com/invoice-template-us-neat-750px.png",
   ...:     "What is the invoice number?"
   ...: )
Out[3]: {'score': 0.9998127222061157, 'answer': 'us-001', 'start': 15, 'end': 15}

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Was this discussed/approved via a Github issue or the forum? Please add a link
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• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@NielsRogge

[HuggingFaceDocBuilderDev]

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

[ankrgyl]

Gentle nudge @Narsil @NielsRogge

[NielsRogge]

Thanks for fixing!