V4L/DVB (6173a): Documentation: Remove reference to dead "cpia_pp=" boot-time option

Since this boot-time option was removed in commit
9ab7e323, delete the reference to it.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>