add doc comment

islandora.install

@@ -56,12 +56,15 @@ function islandora_update_8003(&$sandbox) {
     ->dropTable('islandora_version_count');
 }
 
+/**
+ * Renames migration source keys -> ids.
+ */
 function islandora_update_8004() {
   $config_factory = \Drupal::configFactory();
   $config = $config_factory->getEditable('migrate_plus.migration.islandora__tags');
   if ($config) {
     if (!$config->get('source.ids')) {
-      $config->set('source.ids', $config->get('source.keys');
+      $config->set('source.ids', $config->get('source.keys'));
       $config->clear('source.keys');
       $config->save(TRUE);
     }