acpidump(8): fix usage message

[Michael Reed]

This brings the usage info in line with most other programs.



Index: acpidump.c
===================================================================
RCS file: /cvs/src/usr.sbin/acpidump/acpidump.c,v
retrieving revision 1.13
diff -u -p -r1.13 acpidump.c
--- acpidump.c  3 Oct 2015 01:05:12 -0000       1.13
+++ acpidump.c  3 Oct 2015 06:28:57 -0000
@@ -560,7 +560,7 @@ usage(void)
 {
        extern char     *__progname;
 
-       fprintf(stderr, "%s -o prefix_for_output\n", __progname);
+       fprintf(stderr, "usage: %s -o prefix_for_output\n", __progname);
        exit(1);
 }