libbpf: Correct bpf_core_read.h comment wrt bpf_core_relo struct

Past commit ([0]) removed the last vestiges of struct bpf_field_reloc,
it's called struct bpf_core_relo now.

  [0] 28b93c64

 ("libbpf: Clean up and improve CO-RE reloc logging")

Signed-off-by: Dima Tisnek <dimaqq@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20240121060126.15650-1-dimaqq@gmail.com