fix usage message of pptpd to reflect actual options supported.

Reference: /n/sources/patch/applied/pptp-usage
Date: Wed Sep 29 16:18:34 CES 2010
Signed-off-by: steve@quintile.net

--- /sys/src/cmd/ip/pptp.c	Wed Sep 29 16:18:21 2010
+++ /sys/src/cmd/ip/pptp.c	Wed Sep 29 16:18:18 2010
@@ -54,7 +54,7 @@
 void
 usage(void)
 {
-	fprint(2, "usage: ip/pptp [-Pd] [-s user:secret] [-x pppnetmntpt] [-w window] server\n");
+	fprint(2, "usage: ip/pptp [-Pd] [-k keyspec] [-x pppnetmntpt] [-w window] server\n");
 	exits("usage");
 }