bpf, docs: BPF_FROM_BE exists as alias for BPF_TO_BE

According to include/uapi/linux/bpf.h:

  #define BPF_FROM_LE	BPF_TO_LE
  #define BPF_FROM_BE	BPF_TO_BE

BPF_FROM_BE exists as alias for BPF_TO_BE instead of BPF_TO_LE.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/1651139754-4838-3-git-send-email-yangtiezhu@loongson.cn