docs: use this.setState functional style in example (#485)

Author: lukebrowne

README.md

@@ -73,9 +73,9 @@ class SortableComponent extends Component {
     items: ['Item 1', 'Item 2', 'Item 3', 'Item 4', 'Item 5', 'Item 6'],
   };
   onSortEnd = ({oldIndex, newIndex}) => {
-    this.setState({
-      items: arrayMove(this.state.items, oldIndex, newIndex),
-    });
+    this.setState(({items}) => ({
+      items: arrayMove(items, oldIndex, newIndex),
+    }));
   };
   render() {
     return <SortableList items={this.state.items} onSortEnd={this.onSortEnd} />;