[Security] Fix typo in handler success customization part.

[juferchaud]

Backslashes were missing. Reading was difficult, especially at the end of that very complete page 😅 .

[javiereguiluz]

@juferchaud thanks for this contribution! While merging we removed the .. code-block:: php because we don't use that code-block directive when the code is PHP. The trick we use is to add :: to the paragraph before the code block. That applies the default highlighting to the code, which is configured as PHP in Symfony Docs. Cheers!