fix typo

Reference: /n/atom/patch/applied2013/getexpnit
Date: Fri Oct 25 01:30:37 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/auth/lib/getexpiration.c	Fri Oct 25 01:30:34 2013
+++ /sys/src/cmd/auth/lib/getexpiration.c	Fri Oct 25 01:30:34 2013
@@ -13,7 +13,7 @@
 	Tm date;
 	int i;
 
-	memset(&data, 0, sizeof date);
+	memset(&date, 0, sizeof date);
 	for(i = 0; i < 8; i++)
 		if(!isdigit(d[i]))
 			return date;