docs, af_xdp: Consistent indentation in examples

Examples in this document use all kinds of indentation from 3 to 5
spaces and even mixed with tabs. Making them all even and equal to
4 spaces.

Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20210622185647.3705104-1-i.maximets@ovn.org