docs: Update overview example using format and compile

docs/index.rst

@@ -26,7 +26,7 @@ Blueprint is a markup language and compiler for GTK 4 user interfaces.
 
    using Gtk 4.0;
 
-   template MyAppWindow : ApplicationWindow {
+   template $MyAppWindow: ApplicationWindow {
      default-width: 600;
      default-height: 300;
      title: _("Hello, Blueprint!");
@@ -35,7 +35,7 @@ Blueprint is a markup language and compiler for GTK 4 user interfaces.
      HeaderBar {}
 
      Label {
-       label: bind MyAppWindow.main_text;
+       label: bind template.main_text;
      }
    }