samples/bpf: fix tcp_bpf.readme detach command

Copy-paste, should be detach, not attach.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>