Remove incorrect return type.

src/Core/PluginInterface.php

@@ -89,7 +89,7 @@ public function middleware($middleware);
      * can override this method to replace that behavior.
      *
      * @param \Cake\Routing\RouteBuilder $routes The route builder to update.
-     * @return \Cake\Routing\RouteBuilder
+     * @return void
      */
     public function routes($routes);