Update tutorial-hello-world.md

[parasdaftary]

According other modules Plugin has Router folder for RouterProviderPlugin class instead of Provider folder. I would request to update this to Router folder to be consistent. I tested this and it worked fine.

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[parasdaftary]

If this pull request gets approved then it would require to change references in code sample in documentation for HelloWorld example to Router folder.

namespace Pyz\Yves\HelloWorld\Plugin\Provider - existing

namespace Pyz\Yves\HelloWorld\Plugin\Router - change request