Fix formula string truncated at the end.

Wrong use of substr() in commit 453f8f1821 makes
formula string truncated at the end.

Signed-off-by: Toha <tohenk@yahoo.com>

src/PhpSpreadsheet/Writer/Xls/Parser.php

@@ -563,7 +563,7 @@ class Parser
     private function convertString($string)
     {
         // chop away beggining and ending quotes
-        $string = substr($string, 1, -2);
+        $string = substr($string, 1, -1);
         if (strlen($string) > 255) {
             throw new WriterException('String is too long');
         }