Add typing for children of react-document-title

[marisbest2]

react-document-title uses React.Children.only to get the only child, and raises an error if more than one child is passed.

https://github.com/gaearon/react-document-title/blob/master/index.js#L31

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[typescript-bot]

@marisbest2 Thank you for submitting this PR!

🔔 @cleverguy25 - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

If no reviewer appears after a week, a DefinitelyTyped maintainer will review the PR instead.

[typescript-bot]

After 5 days, no one has reviewed the PR 😞. A maintainer will be reviewing the PR in the next few days and will either merge it or request revisions. Thank you for your patience!