minor #18445 Update cache.rst (tacman)

This PR was submitted for the 6.3 branch but it was merged into the 6.2 branch instead.

Discussion
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Update cache.rst

add 'private' for php 8 constructor

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Commits
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05a70ae Update cache.rst

Author: OskarStark

cache.rst

@@ -315,7 +315,7 @@ with either :class:`Symfony\\Contracts\\Cache\\CacheInterface` or
     }
 
     // in a service
-    public function __construct(CacheInterface $customThingCache)
+    public function __construct(private CacheInterface $customThingCache)
     {
         // ...
     }