correct usage

Reference: /n/atom/patch/applied2013/ramfsusage
Date: Tue Jun 18 22:02:48 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/ramfs.c	Tue Jun 18 22:02:45 2013
+++ /sys/src/cmd/ramfs.c	Tue Jun 18 22:02:45 2013
@@ -999,6 +999,6 @@
 void
 usage(void)
 {
-	fprint(2, "usage: %s [-Dipsu] [-m mountpoint] [-S srvname]\n", argv0);
+	fprint(2, "usage: %s [-Dipsu] [-l memlimit] [-m mntpt] [-S srvname]\n", argv0);
 	exits("usage");
 }