Add forgotten reveal_type import in documentation

[Sukonnik-Illia]

In my opinion documentation reader should see all library imports in code examples even if they are not supposed to be runnable.

[hauntsaninja]

Thanks, but as a convenience feature for dev, mypy will actually understand reveal_type even if it's not imported

In fact, the runtime symbol only exists in typing module in 3.11 onwards so this PR might actually confuse / not work for users on older Pythons