isdn: mISDN: remove a bogus reference to a non-existing doc

The mISDN driver was added on those commits:

	960366cf ("Add mISDN DSP")
	1b2b03f8 ("Add mISDN core files")
	04578dd3 ("Define AF_ISDN and PF_ISDN")
	e4ac9bc1 ("Add mISDN driver")

None of them added a Documentation/isdn/mISDN.cert file.
Also, whatever were supposed to be written there on that time,
probably doesn't make any sense nowadays, as I doubt isdn would
have any massive changes.

So, let's just get rid of the broken reference, in order to
shut up a warning produced by ./scripts/documentation-file-ref-check.
Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

drivers/isdn/mISDN/dsp_core.c

@@ -4,8 +4,6 @@
  *		Karsten Keil (keil@isdn4linux.de)
  *
  *		This file is (c) under GNU PUBLIC LICENSE
- *		For changes and modifications please read
- *		../../../Documentation/isdn/mISDN.cert
  *
  * Thanks to    Karsten Keil (great drivers)
  *              Cologne Chip (great chips)