bug #4744 [Book][Security] Update code example to fit description (xelaris)

This PR was merged into the 2.6 branch.

Discussion
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[Book][Security] Update code example to fit description

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | >=2.6
| Fixed tickets |

The text mentions an `$encoder` object, which wasn't created in the code example.

In addition, I think the paragraph
> The ``$encoder`` object also has an ``isPasswordValid`` method, which takes
> the ``User`` object as the first argument and the plain password to check
> as the second argument.

is also applicable to the 2.3 version (https://github.com/symfony/symfony-docs/blob/2.3/book/security.rst#dynamically-encoding-a-password), where the code example already provides an `$encoder` variable. Should I add it there?

Commits
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bd34c6b Update code example to fit description

Author: weaverryan

book/security.rst

@@ -1174,8 +1174,8 @@ in the following way from a controller::
     // whatever *your* User object is
     $user = new AppBundle\Entity\User();
     $plainPassword = 'ryanpass';
-    $encoded = $this->container->get('security.password_encoder')
-        ->encodePassword($user, $plainPassword);
+    $encoder = $this->container->get('security.password_encoder');
+    $encoded = $encoder->encodePassword($user, $plainPassword);
 
     $user->setPassword($encoded);