minor #16555 [Security] Update firewall_restriction.rst (issamkhadiri1989)

This PR was submitted for the 6.0 branch but it was merged into the 5.4 branch instead.

Discussion
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[Security] Update firewall_restriction.rst

hello,

i may suggest this correction:

the correction assumes the the custom request matcher is **App\Security\CustomRequestMatcher**.

the `request_matcher` option is the service id. it should be mentionned before that  the `app.firewall.secured_area.request_matcher` is the service ID / or an alias like the following :

```
     app.firewall.secured_area.request_matcher:
        alias: App\Security\RequestMatcher
```

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Commits
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e0f53e9 Update firewall_restriction.rst

Author: javiereguiluz

security/firewall_restriction.rst

@@ -215,7 +215,7 @@ If the above options don't fit your needs you can configure any service implemen
         security:
             firewalls:
                 secured_area:
-                    request_matcher: app.firewall.secured_area.request_matcher
+                    request_matcher: App\Security\CustomRequestMatcher
                     # ...
 
     .. code-block:: xml