media: admin-guide: bt8xx.rst: fix a broken cross-reference

The old cards.rst file doesn't exist anymore. Also, it is not
the right reference there, as it should be pointing to the
bttv-specific cardlist.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/admin-guide/media/bt8xx.rst

@@ -18,7 +18,7 @@ General information
 This class of cards has a bt878a as the PCI interface, and require the bttv driver
 for accessing the i2c bus and the gpio pins of the bt8xx chipset.
 
-Please see :doc:`cards` for a complete list of Cards based on the
+Please see :doc:`bttv-cardlist` for a complete list of Cards based on the
 Conexant Bt8xx PCI bridge supported by the Linux Kernel.
 
 In order to be able to compile the kernel, some config options should be