[Mailer] Fix typo

Author: fabpot

Exception/UnsupportedHostException.php

@@ -54,7 +54,7 @@ public function __construct(Dsn $dsn)
         }
         $package = self::HOST_TO_PACKAGE_MAP[$provider] ?? null;
         if ($package && !class_exists($package['class'])) {
-            parent::__construct(sprintf('Unable to send emails via "%s" as the bridge is not installed. Try running "composer require %s".', $host, $package['package']));
+            parent::__construct(sprintf('Unable to send emails via "%s" as the bridge is not installed. Try running "composer require %s".', $provider, $package['package']));
 
             return;
         }