Made down offset a constant.

Change-Id: Ic0e3aa3867dbdce4037cbac23dc26999487c1489
diff --git a/y2016/constants.h b/y2016/constants.h
index 7416a3f..c87559d 100644
--- a/y2016/constants.h
+++ b/y2016/constants.h
@@ -99,6 +99,7 @@
   };
   Wrist wrist;
 
+  const double down_error;
   const char *vision_name;
 };