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>

samples/bpf/sock_example.c

@@ -9,10 +9,10 @@
  *   if (value)
  *        (*(u64*)value) += 1;
  *
- * - attaches this program to eth0 raw socket
+ * - attaches this program to loopback interface "lo" raw socket
  *
  * - every second user space reads map[tcp], map[udp], map[icmp] to see
- *   how many packets of given protocol were seen on eth0
+ *   how many packets of given protocol were seen on "lo"
  */
 #include <stdio.h>
 #include <unistd.h>