xsrftoken: fix lint error in Timeout's comment.

Change-Id: I0c787256e5bae7796fb4b62253a43363b32313f2
Reviewed-on: https://go-review.googlesource.com/17790
Reviewed-by: David Symonds <dsymonds@golang.org>

xsrftoken/xsrf.go

@@ -16,7 +16,7 @@ import (
 	"time"
 )
 
-// The duration that XSRF tokens are valid.
+// Timeout is the duration for which XSRF tokens are valid.
 // It is exported so clients may set cookie timeouts that match generated tokens.
 const Timeout = 24 * time.Hour