samples/bpf: Trivial - fix spelling mistake in usage

Fix spelling mistake.
Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20191007082636.14686-1-anton.ivanov@cambridgegreys.com

samples/bpf/xdpsock_user.c

@@ -378,7 +378,7 @@ static void usage(const char *prog)
 		"  -q, --queue=n	Use queue n (default 0)\n"
 		"  -p, --poll		Use poll syscall\n"
 		"  -S, --xdp-skb=n	Use XDP skb-mod\n"
-		"  -N, --xdp-native=n	Enfore XDP native mode\n"
+		"  -N, --xdp-native=n	Enforce XDP native mode\n"
 		"  -n, --interval=n	Specify statistics update interval (default 1 sec).\n"
 		"  -z, --zero-copy      Force zero-copy mode.\n"
 		"  -c, --copy           Force copy mode.\n"