media: ca-fopen.rst: Fixes the device node name for CA

The device node name for CA is wrong since ever. I suspect
that the name there was before DVBv3 (with was the first API
introduced at the Kernel).

Anyway, use the right name there.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/uapi/dvb/ca-fopen.rst

@@ -49,8 +49,8 @@ Arguments
 Description
 -----------
 
-This system call opens a named ca device (e.g. /dev/ost/ca) for
-subsequent use.
+This system call opens a named ca device (e.g. ``/dev/dvb/adapter?/ca?``)
+for subsequent use.
 
 When an open() call has succeeded, the device will be ready for use. The
 significance of blocking or non-blocking mode is described in the