docs: fix dataclass indentation in expandable Python example

[DeepanshuPal]

Fixes #7715.

The expandable Python example raises IndentationError because the five Book dataclass fields are at column zero instead of inside the class.

Indents those fields without changing their types, defaults or the rest of the example. The original fails Python parsing; the corrected example compiles, runs main(), and passes add/search/checkout/return smoke tests. Mint build validation and git diff --check pass. The broken-link check did not finish within a bounded 25-second run, so I have not verified it.

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Note

Low Risk
Documentation-only whitespace fix in a code sample; no runtime or product behavior changes.

Overview
Fixes the expandable Python sample in create/code.mdx so the Book @dataclass fields are indented under the class instead of at column zero.

That indentation mistake made the snippet invalid Python (IndentationError); only whitespace changes—field types, defaults, and the rest of the library example are unchanged.

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