docs: Fix gdbus-codegen example XML

Without the surrounding <node> tags, gdbus-codegen does not generate any
implementation for the interface described in the introspection XML.

https://bugzilla.gnome.org/show_bug.cgi?id=669544

docs/reference/gio/gdbus-codegen.xml

@@ -420,6 +420,7 @@ gdbus-codegen --c-namespace MyApp                           \
     Consider the following D-Bus Introspection XML.
   </para>
   <informalexample><programlisting><![CDATA[
+<node>
   <interface name="net.Corp.MyApp.Frobber">
     <method name="HelloWorld">
       <arg name="greeting" direction="in" type="s"/>
@@ -434,6 +435,7 @@ gdbus-codegen --c-namespace MyApp                           \
 
     <property name="Verbose" type="b" access="readwrite"/>
   </interface>
+</node>
 ]]></programlisting>
   </informalexample>
   <para>