minor #27545 Update Finder.php (tarlepp)

This PR was submitted for the master branch but it was merged into the 2.8 branch instead (closes #27545).

Discussion
----------

Update Finder.php

Corrected return type which causes following error with [psalm](https://getpsalm.org/)
```
ERROR: PossiblyInvalidArgument - src/Command/Utils/CheckVendorDependencies.php:170:62 - Argument 1 of iterator_to_array expects Traversable, possibly different type array<mixed, Symfony\Component\Finder\SplFileInfo>|Iterator provided $directories = array_map($closure, iterator_to_array($finder->getIterator()));
```

| Q             | A
| ------------- | ---
| Branch?       | 4.1 <!-- see below -->
| Bug fix?      | yes (just docblock bug - nothing serious)
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | -  <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | - <!-- required for new features -->

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Commits
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1100c74 Update Finder.php

Author: nicolas-grekas

Finder.php

@@ -668,7 +668,7 @@ public function in($dirs)
      *
      * This method implements the IteratorAggregate interface.
      *
-     * @return \Iterator|SplFileInfo[] An iterator
+     * @return \Iterator|\SplFileInfo[] An iterator
      *
      * @throws \LogicException if the in() method has not been called
      */