samples/bpf: Add missing option to xdp_sample_pkts usage

xdp_sample_pkts usage() is missing the introduction of the
"-S" option, this patch adds it.

Fixes: d50ecc46 ("samples/bpf: Attach XDP programs in driver mode by default")
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Link: https://lore.kernel.org/bpf/20210615135724.29528-1-wanghai38@huawei.com

samples/bpf/xdp_sample_pkts_user.c

@@ -103,7 +103,8 @@ static void usage(const char *prog)
 	fprintf(stderr,
 		"%s: %s [OPTS] <ifname|ifindex>\n\n"
 		"OPTS:\n"
-		"    -F    force loading prog\n",
+		"    -F    force loading prog\n"
+		"    -S    use skb-mode\n",
 		__func__, prog);
 }