fix name of the Yaml component

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | related to #4189

The component name that is used in the code as well as in the issue tracker used by the symfony/symfony repository is Yaml (instead of YAML). Therefore, this should also be the name being used in the documentation.