doas: add the -n flag to usage string

[Theo Buehler]

A small oversight.  The man page is already up to date.

Index: doas.c
===================================================================
RCS file: /cvs/src/usr.bin/doas/doas.c,v
retrieving revision 1.27
diff -u -p -r1.27 doas.c
--- doas.c      26 Jul 2015 22:44:33 -0000      1.27
+++ doas.c      27 Jul 2015 09:21:38 -0000
@@ -36,7 +36,7 @@
 static void __dead
 usage(void)
 {
-       fprintf(stderr, "usage: doas [-s] [-C config] [-u user] command 
[args]\n");
+       fprintf(stderr, "usage: doas [-ns] [-C config] [-u user] command 
[args]\n");
        exit(1);
 }