Sortable: Fix PHPDoc for parameter `$direction'

`$direction' must be a string instead of an integer because
the direction constants `SORT_ASC' and `SORT_DESC' are strings.

library/Icinga/Data/BaseQuery.php

@@ -105,7 +105,7 @@ abstract class BaseQuery implements QueryInterface, Queryable
      * </code>
      *
      * @param  string   $field
-     * @param  int      $direction
+     * @param  string   $direction
      *
      * @return self
      */

library/Icinga/Data/Sortable.php

@@ -26,7 +26,7 @@ interface Sortable
      * </code>
      *
      * @param  string   $field
-     * @param  int      $direction
+     * @param  string   $direction
      *
      * @return self
      */