fix: add return typing

Signed-off-by: Vitor Mattos <1079143+vitormattos@users.noreply.github.com>

Author: vitormattos

src/Sign/JSignService.php

@@ -21,7 +21,7 @@ public function __construct()
         $this->fileService = JSignFileService::instance();
     }
 
-    public function sign(JSignParam $params)
+    public function sign(JSignParam $params): string
     {
         try {
             $this->validation($params);