vidcontrol: correct history size error message

`vidcontrol -h 0` is acceptable, so be explicit that it's less than zero
that is not allowed.

Reported by:	Siva Mahadevan
Sponsored by:	The FreeBSD Foundation

usr.sbin/vidcontrol/vidcontrol.c

@@ -1359,7 +1359,7 @@ set_history(char *opt)
 
 	if ((*opt == '\0') || size < 0) {
 		revert();
-		errx(1, "argument must be a positive number");
+		errx(1, "argument must not be less than zero");
 	}
 
 	if (ioctl(0, CONS_HISTORY, &size) == -1) {