fixed the usage message to include -k keyspec.

Reference: /n/sources/patch/applied/ftpfs-usage
Date: Fri Sep 17 09:12:09 CES 2010
Signed-off-by: steve@quintile.net

--- /sys/src/cmd/ip/ftpfs/ftpfs.c	Fri Sep 17 01:24:03 2010
+++ /sys/src/cmd/ip/ftpfs/ftpfs.c	Fri Sep 17 01:24:00 2010
@@ -89,7 +89,7 @@
 void
 usage(void)
 {
-	fprint(2, "ftpfs [-/dqnt] [-a passwd] [-m mountpoint] [-e ext] [-o os] [-r root] [net!]address\n");
+	fprint(2, "ftpfs [-/dqnt] [-a passwd] [-m mountpoint] [-e ext] [-k keyspec] [-o os] [-r root] [net!]address\n");
 	exits("usage");
 }