docs: cgroup: Fix stale source file paths

Update two references to files that were moved:
- kernel/cgroup.c -> kernel/cgroup/cgroup.c
- tools/cgroup/cgroup_event_listener.c ->
  samples/cgroup/cgroup_event_listener.c

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Costa Shulyupin <costa.shul@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>