doc: use correct debugfs mountpoint

Use the default mountpoint of debugfs in the pktcdvd ABI.

Signed-off-by: Shen Feng <shen@cn.fujitsu.com>
Cc: <balagi@justmail.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>