Fix link in CachingIterator::key() doc block

src/Model/CachingIterator.php

@@ -76,7 +76,7 @@ class CachingIterator implements Countable, Iterator
     }
 
     /**
-     * @see http://php.net/iterator.mixed
+     * @see http://php.net/iterator.key
      * @return mixed
      */
     public function key()