[Doc] Fixed a minor syntax issue

[javiereguiluz]

The docs of this bundle are now online: https://symfony.com/bundles/NelmioCorsBundle/current/index.html

I spotted a minor syntax issue and this PR fixes it. Thanks!

A note for people not used to RST syntax. Code blocks are usually defined like this:

.. code-block:: php

    // ...

However, since PHP examples are so common in Symfony Docs, we use a trick which consists of this:

Any normal text here, like a paragraph::

    // ... PHP code here

More normal text here [...]

The two :: at the end of any text means that the following indented content is a PHP code block.

[Seldaek]

Nice!