docs: fix Pydantic v2 JSON output in quickstart

[dajiaohuang]

Problem

The quickstart's XML-to-JSON example calls company.json(indent=4). On the Pydantic v2 branch, executing that snippet after loading the documented Company example raises:

TypeError: `dumps_kwargs` keyword arguments are no longer supported.

No JSON output is written. Use Pydantic v2's model_dump_json(indent=4) instead; no library behavior changes.

Validation

• Executed the quickstart JSON code block with the documented Company model and example XML: original failed with the exception above; corrected snippet writes JSON matching the parsed model.
• Public examples: 49 passed with lxml; 48 passed and 1 backend-specific skip with ElementTree (Python 3.13, Pydantic 2.14.1, lxml 6.1.3). The binary-input custom-encoder example used the repository's committed LF fixture bytes rather than Windows checkout CRLF conversion.
• Synthetic public-API roundtrips for namespaced attributes/elements, generics, a field validator, a discriminated union and an in-memory XML tree passed on both backends.
• Sphinx 5.3 HTML build passed on Python 3.12 with intersphinx inventories disabled; external-reference availability was not checked.
• git diff --check passed. Full test/version matrix and CI were not run locally.