docs: initialize the input in the unflatten example

[Likio3000]

The unflatten README example calls devalue.stringify(data) before declaring const data later in the same code block. Running the example in one scope therefore throws a ReferenceError before serialization.

Define a small input value to serialize and keep data for the revived result, with an expected-output comment to make the example self-contained.

Validation: static review of JavaScript lexical binding and the stringify/unflatten signatures in src/stringify.js and src/parse.js. This changes only the README; I did not run the project suite or install dependencies.

Prepared with AI assistance (OpenAI Codex), with source and diff review.

[changeset-bot]

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[Rich-Harris]

thanks