xsk: sample kernel code is now in libbpf

Fix documentation that mention xdpsock_kern.c which has been
replaced by code embedded in libbpf.

Signed-off-by: Eric Leblond <eric@regit.org>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>