minor symfony#21250 Fix a typo (GuilhemN)

This PR was merged into the 3.2 branch.

Discussion
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Fix a typo

| Q             | A
| ------------- | ---
| Branch?       | 3.2
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#21194 (comment)
| License       | MIT
| Doc PR        |

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Commits
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2dcb22e Fix a typo

Author: nicolas-grekas

UPGRADE-4.0.md

@@ -250,7 +250,7 @@ Validator
    ```
 
  * The default value of the strict option of the `Choice` Constraint has been
-   changed to `true` as of 4.0. If you need the the previous behaviour ensure to 
+   changed to `true` as of 4.0. If you need the previous behaviour ensure to 
    set the option to `false`.
 
 Yaml