Fix user/OS dropdown type migration

Author: stonebuzz

inc/field.class.php

@@ -108,14 +108,15 @@ public static function install(Migration $migration, $version)
             $DB->buildUpdate(
                 PluginFieldsField::getTable(),
                 ['type' => 'dropdown-User'],
-                ['type' => 'dropdownusers']
+                ['type' => 'dropdownuser']
             )
         );
+
         $migration->addPostQuery(
             $DB->buildUpdate(
                 PluginFieldsField::getTable(),
-                ['type' => 'dropdown-User'],
-                ['type' => 'dropdownusers']
+                ['type' => 'dropdown-OperatingSystem'],
+                ['type' => 'dropdownoperatingsystems']
             )
         );