bpf, docs: BPF Iterator Document

Fix the description of the seq_info field of the bpf_iter_reg structure which
was wrong due to an accidental copy/paste of the previous field's description.

Fixes: 8972e18a ("bpf, docs: BPF Iterator Document")
Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20230622095407.1024053-1-aspsk@isovalent.com