libbpf: Fix map index used in error message

The error message emitted by bpf_object__init_user_btf_maps() was using the
wrong section ID.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200819110534.9058-1-toke@redhat.com