[changed] Update overview.md

Tiny swap to bring docs up to date.

Author: imjared

docs/guides/overview.md

@@ -371,7 +371,7 @@ var App = React.createClass({
   getHandlerKey: function () {
     var childDepth = 1; // assuming App is top-level route
     var { router } = this.context;
-    var key = router.getRoutes()[childDepth].name;
+    var key = router.getCurrentRoutes()[childDepth].name;
     var id = router.getParams().id;
     if (id) { key += id; }
     return key;