docs: fix typo in migration guide

[safwansamsudeen]

This is how the guide looks right now:

I removed the extra ?. - is it necessary?

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Latest commit: ffabc14

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[leonardsimonse]

The docs are correct in this instance. children is of type 'any', so this notation is required. It ensures children is not null or undefined. children might not always be passed as a prop and the component should not crash in that instance.