Fixed variable name (changed unintentionally in last backporting commit)

config/main.inc.php.dist

@@ -437,7 +437,7 @@ $rcmail_config['date_format'] = 'Y-m-d';
 
 // give this choice of date formats to the user to select from
 // Note: do not use ambiguous formats like m/d/Y
-$config['date_formats'] = array('Y-m-d', 'Y/m/d', 'Y.m.d', 'd-m-Y', 'd/m/Y', 'd.m.Y', 'j.n.Y');
+$rcmail_config['date_formats'] = array('Y-m-d', 'Y/m/d', 'Y.m.d', 'd-m-Y', 'd/m/Y', 'd.m.Y', 'j.n.Y');
 
 // use this format for time display (date or strftime format)
 $rcmail_config['time_format'] = 'H:i';