[docs] [issue35745] Add import statement in dataclass code snippet

[Windson Yang]

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

I'm not sure if we should put 

from dataclasses import dataclass

everywhere or we should put it just in the first example as I did in the PR.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35745>
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