minor #28310 [Finder] Update PHPdoc append() (ro0NL)

This PR was squashed before being merged into the 2.8 branch (closes #28310).

Discussion
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[Finder] Update PHPdoc append()

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | #...   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

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Commits
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111d9bc6f5 [Finder] Update PHPdoc append()

src/Symfony/Component/Finder/Finder.php

@@ -699,7 +699,7 @@ class Finder implements \IteratorAggregate, \Countable
      *
      * The set can be another Finder, an Iterator, an IteratorAggregate, or even a plain array.
      *
-     * @param mixed $iterator
+     * @param iterable $iterator
      *
      * @return $this
      *
@@ -751,7 +751,7 @@ class Finder implements \IteratorAggregate, \Countable
     }
 
     /**
-     * @param $dir
+     * @param string $dir
      *
      * @return \Iterator
      */