qom: Fix typo, 'my_class_init' -> 'derived_class_init'

Signed-off-by: NZhoujian <jianjay.zhou@huawei.com>
Signed-off-by: NGonglei <arei.gonglei@huawei.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>

include/qom/object.h

@@ -273,7 +273,7 @@ typedef struct InterfaceInfo InterfaceInfo;
  *     .name = TYPE_DERIVED,
  *     .parent = TYPE_MY,
  *     .class_size = sizeof(DerivedClass),
- *     .class_init = my_class_init,
+ *     .class_init = derived_class_init,
  * };
  *   </programlisting>
  * </example>