Add missing %s so that all of the usage message gets printed.

Bump WARNS to 6 while I'm here.

usr.bin/id/Makefile

@@ -2,6 +2,7 @@
 # $FreeBSD$
 
 PROG=	id
+WARNS?=	6
 LINKS=	${BINDIR}/id ${BINDIR}/groups
 LINKS+=	${BINDIR}/id ${BINDIR}/whoami
 MAN=	id.1 groups.1 whoami.1

usr.bin/id/id.c

@@ -401,7 +401,7 @@ usage(void)
 	else if (iswhoami)
 		(void)fprintf(stderr, "usage: whoami\n");
 	else
-		(void)fprintf(stderr, "%s\n%s\n%s\n%s\n%s\n%s\n",
+		(void)fprintf(stderr, "%s\n%s\n%s\n%s\n%s\n%s\n%s\n",
 		    "usage: id [user]",
 		    "       id -G [-n] [user]",
 		    "       id -M",