update parsedown.php

Closes #3504206

src/Plugin/Markdown/Parsedown/Parsedown.php

@@ -62,7 +62,7 @@ class Parsedown extends BaseParser implements AllowedHtmlInterface, SettingsInte
   /**
    * {@inheritdoc}
    */
-  public function __sleep() {
+  public function __sleep() : array {
     unset($this->parsedown);
     return parent::__sleep();
   }