Minor - fix php warning

Author: jmontoyaa

main/inc/lib/certificate.lib.php

@@ -332,7 +332,7 @@ public function generate($params = [], $sendNotification = false)
             $my_new_content_html = str_replace(
                 '((certificate_barcode))',
                 Display::img(
-                    +$this->certification_web_user_path.$file_info['filename'].'_qr.png',
+                    $this->certification_web_user_path.$file_info['filename'].'_qr.png',
                     'QR'
                 ),
                 $content