fix: less strict prop type so that it works inside SSR (#92)

Author: DRoet

src/ScrollSyncPane.js

@@ -28,8 +28,7 @@ export default class ScrollSyncPane extends Component {
     enabled: PropTypes.bool,
     innerRef: PropTypes.oneOfType([// Either a function
       PropTypes.func,
-    // Or the instance of a DOM native element (see the note about SSR)
-      PropTypes.shape({ current: PropTypes.instanceOf(Element) })])
+      PropTypes.shape({ current: PropTypes.any })])
   };
 
   static defaultProps = {