minor #17717 fixed codeblock in UPGRADE-3.1.md (eXtreme)

This PR was merged into the 3.1-dev branch.

Discussion
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fixed codeblock in UPGRADE-3.1.md

Missing backticks break upgrade notes for 3.1.

| Q             | A
| ------------- | ---
| Bug fix?      | -
| New feature?  | -
| BC breaks?    | -
| Deprecations? | -
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
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cb00055 fixed codeblock in UPGRADE-3.1.md

UPGRADE-3.1.md

@@ -45,6 +45,7 @@ Yaml
 
    ```php
    Yaml::dump(array('foo' => new A(), 'bar' => 1), 0, 0, false, Yaml::DUMP_OBJECT);
+   ```
 
  * The `!!php/object` tag to indicate dumped PHP objects has been deprecated
    and will be removed in Symfony 4.0. Use the `!php/object` tag instead.