[gtkmm-documentation] Update Assistant example program

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 0ae7c092be7d0ce23ca75816c46deea127592898
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Thu Jan 21 18:17:18 2021 +0100

    Update Assistant example program

 examples/book/assistant/exampleassistant.cc | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)
---
diff --git a/examples/book/assistant/exampleassistant.cc b/examples/book/assistant/exampleassistant.cc
index 3a867a8..479b6be 100644
--- a/examples/book/assistant/exampleassistant.cc
+++ b/examples/book/assistant/exampleassistant.cc
@@ -24,12 +24,13 @@ ExampleAssistant::ExampleAssistant()
   m_check("Optional extra information")
 {
   set_title("Gtk::Assistant example");
-  set_default_size(400, 300);
+  set_default_size(400, 200);
 
   m_box.append(m_label1);
   m_box.append(m_entry);
-  m_label1.set_expand();
-  m_entry.set_expand();
+  m_label1.set_wrap();
+  m_label1.set_valign(Gtk::Align::CENTER);
+  m_entry.set_valign(Gtk::Align::CENTER);
 
   append_page(m_box);
   append_page(m_check);