Fix the descriptions of the return values of helper bpf_current_task_under_cgroup().

Fixes: c6b5fb86 ("bpf: add documentation for eBPF helpers (42-50)")
Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220310155335.1278783-1-hengqi.chen@gmail.com