Update typechecking-with-proptypes.md

[stinoga]

Shouldn't children's PropType be a node? Here's a good conversation on it in the eslint-plugin-react repo.

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[gaearon]

Thanks for PR! I think it's a bit ambiguous so I'd leave it as is. For example you might want .element if you need a single element. Such as if you use cloneElement() there or if you return it without a wrapping <div> in some cases.