docs: Fix panic in example chapter 1

Also copied from the hello_world example

book/src/chapter_1.md

@@ -20,6 +20,7 @@ fn startup(
     font_mapping.set_default(asset_server.load("roboto.kayak_font"));
 
     let mut widget_context = KayakRootContext::new();
+    widget_context.add_plugin(KayakWidgetsContextPlugin);
     let parent_id = None;
 
     // The rsx! macro expects a parent_id, a widget_context from the user.