Fixed phpdoc (#1689)

Fixed phpdoc of Mage_Core_Model_Resource_Db_Collection_Abstract::addExpressionFieldToSelect

Author: randallelliott714

app/code/core/Mage/Core/Model/Resource/Db/Collection/Abstract.php

@@ -333,13 +333,13 @@ public function addFieldToSelect($field, $alias = null)
 
     /**
      * Add attribute expression (SUM, COUNT, etc)
-     * Example: ('sub_total', 'SUM({{attribute}})', 'revenue')
+     * Example: ('sub_total', 'SUM({{attribute}})', array('attribute' => 'revenue'))
      * Example: ('sub_total', 'SUM({{revenue}})', 'revenue')
      * For some functions like SUM use groupByAttribute.
      *
      * @param string $alias
      * @param string $expression
-     * @param array $fields
+     * @param array|string $fields
      * @return $this
      */
     public function addExpressionFieldToSelect($alias, $expression, $fields)