Fix bad encoding for tidy_repair_string

Tidy on PHP 8.5 is more restrictive on what can be given as input encoding. Before, it worked as an unknown value was converted to `utf8`.

src/Readability.php

@@ -118,7 +118,7 @@ class Readability implements LoggerAwareInterface
         'enclose-text' => true,
         'merge-divs' => true,
         // 'merge-spans' => true,
-        'input-encoding' => '????',
+        'input-encoding' => 'utf8',
         'output-encoding' => 'utf8',
         'hide-comments' => true,
     ];