Fix boolean notation in PHPDoc (#314)

src/Spout/Reader/XLSX/RowIterator.php

@@ -115,7 +115,7 @@ class RowIterator implements IteratorInterface
      * Checks if current position is valid
      * @link http://php.net/manual/en/iterator.valid.php
      *
-     * @return boolean
+     * @return bool
      */
     public function valid()
     {

src/Spout/Reader/XLSX/SheetIterator.php

@@ -53,7 +53,7 @@ class SheetIterator implements IteratorInterface
      * Checks if current position is valid
      * @link http://php.net/manual/en/iterator.valid.php
      *
-     * @return boolean
+     * @return bool
      */
     public function valid()
     {