Update security.rst

Fixing https://github.com/symfony/symfony-docs/pull/13388/checks?check_run_id=524167834

Author: ThomasLandauer

security.rst

@@ -339,7 +339,7 @@ you to control *every* part of the authentication process (see the next section)
     bundle.
 
 Guard Authenticators
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+~~~~~~~~~~~~~~~~~~~~
 
 A Guard authenticator is a class that gives you *complete* control over your
 authentication process. There are many different ways to build an authenticator;