Remove trailing slash in URL

Trailing slash in GitHub URLs is not needed, and for consistency with
PHPMailer logo which does not have the trailing slash

class.phpmailer.php

@@ -1844,7 +1844,7 @@ public function createHeader()
         if ($this->XMailer == '') {
             $result .= $this->headerLine(
                 'X-Mailer',
-                'PHPMailer ' . $this->Version . ' (https://github.com/PHPMailer/PHPMailer/)'
+                'PHPMailer ' . $this->Version . ' (https://github.com/PHPMailer/PHPMailer)'
             );
         } else {
             $myXmailer = trim($this->XMailer);