doc-rst: userp: re-add the missing captions

The conversion from DocBook removed them. Re-add.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/linux_tv/media/v4l/userp.rst

@@ -28,6 +28,7 @@ indexed and cannot be queried like mapped buffers with the
 
 
 .. code-block:: c
+    :caption: Example 3.3. Initiating streaming I/O with user pointers
 
     struct v4l2_requestbuffers reqbuf;