Problem with Composer - PSR-4

I'm studying Composer and its forms of autoload (classmap, file, psr-0, psr-4), however in my example it is appearing with the following error:

Fatal error: Class 'Library\Exemplo' not found in/var/www/html/CursoPHP/composer/index.php on line 5

My example folder structure is this:

/ comp 
   |-- /src 
     |-- /Library 
           |-- exemplo.php
   |-- /vendor 
   |-- index.php 

My index.php:

require_once 'vendor/autoload.php';

$nome = new \Library\Exemplo();

My composer.json:

"autoload": {
    "psr-4": {
      "Library\": "src/Library"
    }
}

My example.php:

<?php namespace Library;

  class Exemplo
  {
    public function __construct()
    {
      echo 'Classe Exemplo Iniciada';
    }
   }

 ?>

With all other means of composer autoload I got success, but I can not find the error of my example using the PSR-4.