doc-rst: control: Fix missing reference for example 8

The conversion broke references and captions for examples.

Fix the missing reference for control enumeration.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/linux_tv/media/v4l/control.rst

@@ -371,6 +371,8 @@ device has one or more controls, ``VIDIOC_QUERYMENU`` when it has one or
 more menu type controls.
 
 
+.. _enum_all_controls:
+
 .. code-block:: c
     :caption: Example 8: Enumerating all user controls