example: Fix up margins

This makes the code match what we show in the screenshots. This
broke when headerbars were changed to allow titles to extend into
the margin.

examples/application7/window.ui

@@ -15,6 +15,7 @@
             <child type="title">
               <object class="GtkStackSwitcher" id="tabs">
                 <property name="visible">True</property>
+                <property name="margin">6</property>
                 <property name="stack">stack</property>
               </object>
             </child>