docs: use |[ ]| instead of <programlisting></programlisting>

https://bugzilla.gnome.org/show_bug.cgi?id=723119

gtk/gtkicontheme.c

@@ -118,8 +118,7 @@
  * so that the icon information is shared with other people
  * looking up icons. In the case where the default screen is
  * being used, looking up an icon can be as simple as:
- * <informalexample>
- * <programlisting>
+ * |[
  * GError *error = NULL;
  * GtkIconTheme *icon_theme;
  * GdkPixbuf *pixbuf;
@@ -140,8 +139,7 @@
  *     // Use the pixbuf
  *     g_object_unref (pixbuf);
  *   }
- * </programlisting>
- * </informalexample>
+ * ]|
  */