laravel中的电子邮件显示的是变量而不是值

namespace App\Mail;

use Illuminate\Bus\Queueable;
use Illuminate\Mail\Mailable;
use Illuminate\Queue\SerializesModels;
use Illuminate\Contracts\Queue\ShouldQueue;

class WelcomeEmail extends Mailable
{
    use Queueable, SerializesModels;

    public $userName;

    /**
     * Create a new message instance.
     *
     * @return void
     */
    public function __construct($userName)
    {
        $this->userName = $userName;
    }

    /**
     * Build the message.
     *
     * @return $this
     */
    public function build()
    {
        return $this->view('emails.welcome')
                    ->subject('Welcome to Our Website');
    }
}

<p>Dear {{ $userName }},</p>
<p>Welcome to our website!</p>

use App\Mail\WelcomeEmail;
use Illuminate\Support\Facades\Mail;

Mail::to('example@example.com')->send(new WelcomeEmail('John'));