bug symfony#49248 [Config] Fix phpdoc nullable (alamirault)

This PR was merged into the 5.4 branch.

Discussion
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[Config] Fix phpdoc nullable

| Q             | A
| ------------- | ---
| Branch?       | 5.4| Bug fix?      | yes/no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | Fix #... <!-- prefix each issue number with "Fix #", no need to create an issue if none exists, explain below instead -->
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->
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Commits
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7865255 [Config] Fix phpdoc nullable

Author: chalasr

src/Symfony/Component/Config/Definition/Processor.php

@@ -63,9 +63,9 @@ public function processConfiguration(ConfigurationInterface $configuration, arra
      *
      * extensions: ['twig.extension.foo', 'twig.extension.bar']
      *
-     * @param array  $config A config array
-     * @param string $key    The key to normalize
-     * @param string $plural The plural form of the key if it is irregular
+     * @param array       $config A config array
+     * @param string      $key    The key to normalize
+     * @param string|null $plural The plural form of the key if it is irregular
      */
     public static function normalizeConfig(array $config, string $key, string $plural = null): array
     {