wrong table in delete revision SQL for synthesized products

reported by coupet (http://drupal.org/user/44538)

See delete FROM {chemical}
table chemical does not exist.

/**
 * Implementation of hook_nodeapi().
 */
function synth_products_nodeapi(&$node, $op, $teaser, $page) {
 switch ($op) {
   case 'delete revision':
     db_query('DELETE FROM {chemical} WHERE vid = %d', $node->vid);
     break;
 }
}