[docs] [issue36661] Missing dataclass decorator import in dataclasses module docs

[Windson Yang]

Windson Yang <wiwindson at outlook.com> added the comment:

I can find some example in the docs that didn't `import the correct module` even in the first example. Should we add the `import` statement for all of them?

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nosy: +Windson Yang

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36661>
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