Update README.md

Author: cheton

README.md

@@ -87,7 +87,7 @@ class App extends React.Component {
                         return true;
                     }}
                     onClick={(event) => {
-                        var target = event.target || event.srcElement; // IE8
+                        const target = event.target || event.srcElement; // IE8
                         console.log('click:', target);
                     }}
                     onOpenNode={(node) => {