docs/devel/qom.rst: Correct code style

Per commit 067109a ("docs/devel: mention the spacing requirement
for QOM"):

  For a storage structure the first declaration should always be
  called “parent_obj” and for a class structure the first member
  should always be called “parent_class”

Adapt the QOM rST document accordingly.

Reported-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230622101717.70468-1-philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230630180423.558337-26-alex.bennee@linaro.org>

docs/devel/qom.rst

@@ -26,7 +26,7 @@ features:
    typedef DeviceClass MyDeviceClass;
    typedef struct MyDevice
    {
-       DeviceState parent;
+       DeviceState parent_obj;
 
        int reg0, reg1, reg2;
    } MyDevice;
@@ -147,7 +147,7 @@ will also have a wrapper function to call it easily:
 
    typedef struct MyDeviceClass
    {
-       DeviceClass parent;
+       DeviceClass parent_class;
 
        void (*frobnicate) (MyDevice *obj);
    } MyDeviceClass;