example: Don't set a nonexisting property

The GtkHeaderBar::show-fallback-app-menu property does not exist,
and isn't needed anyway.

docs/reference/gtk/getting_started.xml

@@ -982,9 +982,8 @@ example_app_window_init (ExampleAppWindow *win)
         <programlisting><xi:include href="../../../../examples/application10/window.ui" parse="text"><xi:fallback>FIXME: MISSING XINCLUDE CONTENT</xi:fallback></xi:include></programlisting>
       </informalexample>
 
-      <para>A small extra refinement that we have added here is to allow
-      the header bar to provide a fallback application menu, by setting
-      the show-fallback-app-menu property to TRUE. Here is how the
+      <para>A small extra bonus of using a header bar is that we get
+      a fallback application menu for free. Here is how the
       application now looks, if this fallback is used.</para>
 
       <informalfigure>

examples/application10/window.ui

@@ -9,7 +9,6 @@
           <object class="GtkHeaderBar" id="header">
             <property name="visible">True</property>
             <property name="show-close-button">True</property>
-            <property name="show-fallback-app-menu">True</property>
             <child>
               <object class="GtkLabel" id="lines_label">
                 <property name="visible">False</property>