cgroup: fix obsolete function name

cgroup_taskset_migrate() has been renamed to cgroup_migrate_execute() since
commit e595cd70 ("cgroup: track migration context in cgroup_mgctx").
Update the corresponding comment.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>