minor symfony#18806 [YAML] Fixed upgrade file (HeahDude)

This PR was merged into the 3.1 branch.

Discussion
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[YAML] Fixed upgrade file

| Q             | A
| ------------- | ---
| Branch?       | 3.1
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

Commits
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6bcb74d [YAML] fixed "dump" signature in upgrade file

Author: xabbuh

UPGRADE-3.1.md

@@ -202,7 +202,7 @@ Yaml
    After:
 
    ```php
-   Yaml::dump(array('foo' => new A(), 'bar' => 1), 0, 0, false, Yaml::DUMP_OBJECT);
+   Yaml::dump(array('foo' => new A(), 'bar' => 1), 0, 0, Yaml::DUMP_OBJECT);
    ```
 
  * The `!!php/object` tag to indicate dumped PHP objects has been deprecated