minor #20301 [SecurityBundle] Changed encoder configuration example to bcrypt (jeremyFreeAgent)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #20301).

Discussion
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[SecurityBundle] Changed encoder configuration example to bcrypt

| Q             | A
| ------------- | ---
| Branch?       | "master"
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Simple change in the configuration example to help developers to not use `sha512` as encoder when using `config:dump-reference`.

Commits
-------

a55058f [SecurityBundle] Changed encoder configuration example to bcrypt

src/Symfony/Bundle/SecurityBundle/DependencyInjection/MainConfiguration.php

@@ -407,11 +407,10 @@ class MainConfiguration implements ConfigurationInterface
             ->children()
                 ->arrayNode('encoders')
                     ->example(array(
-                        'Acme\DemoBundle\Entity\User1' => 'sha512',
+                        'AppBundle\Entity\User1' => 'bcrypt',
-                        'Acme\DemoBundle\Entity\User2' => array(
+                        'AppBundle\Entity\User2' => array(
-                            'algorithm' => 'sha512',
+                            'algorithm' => 'bcrypt',
-                            'encode_as_base64' => 'true',
+                            'cost' => 13,
-                            'iterations' => 5000,
                         ),
                     ))
                     ->requiresAtLeastOneElement()