bug #4145 Fix documentation for group_sequence_provider (giosh94mhz)

This PR was merged into the 2.3 branch.

Discussion
----------

Fix documentation for group_sequence_provider

The current documentation for `group_sequence_provider` option in the docs is wrong.
It states that the option should be set with:
```YAML
Your\Class\Name:
    group_sequence_provider: ~
    ...
````
But this didn't work.

By looking at the unit test I came up the the right version:
```YAML
Your\Class\Name:
    group_sequence_provider: true
    ...
````

Commits
-------

81cec5d Fix documentation for group_sequence_provider

Author: weaverryan

book/validation.rst

@@ -1115,7 +1115,7 @@ provides a sequence of groups to be validated:
 
         # src/Acme/DemoBundle/Resources/config/validation.yml
         Acme\DemoBundle\Entity\User:
-            group_sequence_provider: ~
+            group_sequence_provider: true
 
     .. code-block:: php-annotations