fix(typings): children props should be ReactNode (#72)

ref. https://github.com/DefinitelyTyped/DefinitelyTyped/blob/41fe335d209b4cd46dedafd1a91849d7fec6880d/types/react/index.d.ts#L1218-L1222

Author: polamjag

types/index.d.ts

@@ -1,7 +1,7 @@
 import * as React from 'react';
 
 interface Props {
-  children: React.ReactElement<any> | null | undefined;
+  children: React.ReactNode;
   root?: string | Element | null;
   rootMargin?: string;
   threshold?: number | number[];