minor #13469 Fix docblocks to comments (keradus)

This PR was squashed before being merged into the 2.3 branch (closes #13469).

Discussion
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Fix docblocks to comments

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | ?
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

Change docblock into comment when it's not a proper docblock.

Commits
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779926a Fix docblocks to comments

Author: fabpot

Exception/FatalErrorException.php

@@ -11,7 +11,7 @@
 
 namespace Symfony\Component\HttpKernel\Exception;
 
-/**
+/*
  * Fatal Error Exception.
  *
  * @author Konstanton Myakshin <koc-dp@yandex.ru>

Exception/FlattenException.php

@@ -11,7 +11,7 @@
 
 namespace Symfony\Component\HttpKernel\Exception;
 
-/**
+/*
  * FlattenException wraps a PHP Exception to be able to serialize it.
  *
  * Basically, this class removes all objects from the trace.