fix for typescript

if I don't do this - ide throws error for typescript
#3050

Author: johndpope

README.md

@@ -40,7 +40,7 @@ import "react-datepicker/dist/react-datepicker.css";
 const Example = () => {
   const [startDate, setStartDate] = useState(new Date());
   return (
-    <DatePicker selected={startDate} onChange={(date) => setStartDate(date)} />
+    <DatePicker selected={startDate} onChange={(date:Date) => setStartDate(date)} />
   );
 };
 ```