Documentation: sysctl/kernel.txt: fix documentation reference

This patch fixes a reference to Documentation/kmod.txt
which was apparently renamed to Documentation/debugging-modules.txt

Signed-off-by: Michael Opdenacker <michael@free-electrons.com>
Cc: "Randy.Dunlap" <rdunlap@xenotime.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>