Add missing string and fix upgrade previous value.

db/upgrade.php

@@ -467,7 +467,7 @@ function xmldb_attendance_upgrade($oldversion=0) {
 
     if ($oldversion < 2017050225) {
         $table = new xmldb_table('attendance');
-        $field = new xmldb_field('showextrauserdetails', XMLDB_TYPE_INTEGER, '1', XMLDB_UNSIGNED, XMLDB_NOTNULL, null, '1', 'subnet');
+        $field = new xmldb_field('showextrauserdetails', XMLDB_TYPE_INTEGER, '1', XMLDB_UNSIGNED, XMLDB_NOTNULL, null, '1', 'showsessiondetails');
         if (!$dbman->field_exists($table, $field)) {
             $dbman->add_field($table, $field);
         }

lang/en/attendance.php

@@ -224,6 +224,7 @@ $string['hiddensessions_help'] = 'Sessions are hidden if they are scheduled befo
 
 You can use this feature to hide older sessions instead of deleting them. Only visible sessions will appear in the Gradebook.';
 $string['hiddensessionsdeleted'] = 'All hidden sessions were delete';
+$string['hideextrauserdetails'] = 'Hide extra user details';
 $string['hidensessiondetails'] = 'Hide session details';
 $string['import'] = 'Import';
 $string['importfile'] = 'Import file';