Fix useState types for date time picker example

Author: noahtallen

client/devdocs/design/wordpress-components-gallery/date-time-picker.tsx

@@ -2,7 +2,7 @@ import { DateTimePicker } from '@wordpress/components';
 import { useState } from 'react';
 
 const DateTimePickerExample = () => {
-	const [ dateTime, setDateTime ] = useState( '' );
+	const [ dateTime, setDateTime ] = useState< string | null >( '' );
 
 	return <DateTimePicker currentDate={ dateTime } onChange={ setDateTime } />;
 };