Fix outdated Authentication plugin version and namespace for DefaultPasswordHasher

1) Updated composer command from 

^2.0 

to 

^3.0.

2) Updated DefaultPasswordHasher namespace from 

use Cake\Auth\DefaultPasswordHasher;

to

Authentication\PasswordHasher\DefaultPasswordHasher

Author: kiseragi

ja/tutorials-and-examples/blog-auth-example/auth.rst

@@ -121,7 +121,7 @@ composerを使ってAuthenticationプラグインをインストールします
 
 .. code-block:: console
 
-    composer require "cakephp/authentication:^2.0"
+    composer require "cakephp/authentication:^3.0"
 
 
 パスワードハッシュの追加
@@ -134,7 +134,7 @@ composerを使ってAuthenticationプラグインをインストールします
     // src/Model/Entity/User.php
     namespace App\Model\Entity;
 
-    use Cake\Auth\DefaultPasswordHasher;
+    use Authentication\PasswordHasher\DefaultPasswordHasher;
     use Cake\ORM\Entity;
 
     class User extends Entity