minor #13212 [Yaml] Update README.md (xelaris)

This PR was merged into the 2.3 branch.

Discussion
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[Yaml] Update README.md

| Q             | A
| ------------- | ---
| Fixed tickets |
| License       | MIT

The ability to pass file names to Yaml::parse() was deprecated in 2.2 and will be removed in 3.0. So show an up-to-date example.

Commits
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bab98f0 [Yaml] Update README.md

src/Symfony/Component/Yaml/README.md

@@ -6,7 +6,7 @@ YAML implements most of the YAML 1.2 specification.
 ```php
 use Symfony\Component\Yaml\Yaml;
 
-$array = Yaml::parse($file);
+$array = Yaml::parse(file_get_contents(filename));
 
 print Yaml::dump($array);
 ```