samples/bpf: correct comment in sock_example.c

The program run against loopback interace "lo", not "eth0".
Correct the comment.

Signed-off-by: Wang Sheng-Hui <shhuiw@foxmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>