wrong option name in upgrade.drush.inc

not tested but looks like a mistake:

+++ commands/core/upgrade.drush.inc	28 Jan 2010 20:06:49 -0000
@@ -120,7 +120,7 @@ function drush_upgrade_site_upgrade($tar
   // Copy source database to target database. The source DB is not changed.
   // Always set 'common' at minimum. Sites that want other can create other key in drushrc.php.
   if (!drush_get_option('structure-tables-key')) {
-    drush_set_option('structure_tables-key', 'common');
+    drush_set_option('structure-tables-key', 'common');
   }
   // Always blow away the target database so we start fresh.
   drush_set_option('create-db', TRUE);