minor symfony#57521 [Yaml] Fix typo in changelog (alexandre-daubois)

This PR was merged into the 7.2 branch.

Discussion
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[Yaml] Fix typo in changelog

| Q             | A
| ------------- | ---
| Branch?       | 7.2
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        | -
| License       | MIT

Fix message to be consistent with the rest

Commits
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3772bce [Yaml] Fix typo in changelog

Author: fabpot

src/Symfony/Component/Yaml/CHANGELOG.md

@@ -4,7 +4,7 @@ CHANGELOG
 7.2
 ---
 
- * duplicate mapping keys throw a `ParseException` even when such key has a NULL value
+ * Duplicate mapping keys throw a `ParseException` even when such key is `null`
 
 7.1
 ---