Format examples in presigned urls guide

[jonathan343]

Small PR to format examples in the S3 Presigned URLs dev guide. We should do a follow up to format all examples in our repos.

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Testing

Used the following steps to generate docs and verify they generate correctly:

1. Create a venv using uv venv -p 3.8
2. Activate the venv uv pip install -r requirements-docs.txt
3. Install boto3 uv pip install -e .
4. Change into the docs directory using cd docs
5. Build the docs using make clean html
   • I modified boto3/docs/__init__.py to only generate docs for one service to speed the build up.
6. Change into the directory with generated html files cd build/html
7. Host the html files locally python -m http.server
8. Verify the http://[::]:8000/guide/s3-presigned-urls.html page renders correctly.