fix name of the Yaml component

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.
minor #4199 fix name of the Yaml component (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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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.

Commits
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5187768 fix name of the Yaml component