update plugin/headers config entry

use future/past instead of days:

-headers days 5 reject_type temp require From,Date
+headers reject 1 reject_type temp require From,Date future 2 past 15

config.sample/plugins

@@ -57,7 +57,7 @@ auth/authdeny
 # this plugin needs to run after all other "rcpt" plugins
 rcpt_ok
 
-headers days 5 reject_type temp require From,Date
+headers reject 1 reject_type temp require From,Date future 2 past 15
 dkim
 
 # content filters