Documentation: fix cgroup blkio throttle filenames

All the blkio.throttle.* file names are incorrectly reported without
".throttle" in the documentation. Fix it.

Signed-off-by: Andrea Righi <andrea@betterlinux.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>