Fix usage message (ALT #22651, https://bugzilla.altlinux.org/show_bug.cgi?id=22651)

--- dhcpdump.c
+++ dhcpdump.c.orig
@@ -71,7 +71,7 @@ void	printHex(u_char *data, int len);
 void	printHexString(u_char *data, int len);
 
 void usage() {
-	printf("Usage: $0 <-i interface> [-h macaddress]\n");
+	printf("Usage: dhcpdump -i <interface> [-h <macaddress>]\n");
 	exit(0);
 }