Correct error in "Develop a third party module" docs

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I've reviewed the MR and confirmed that the proposed correction resolves the issue in the documentation.
The line:

$provider_model = $ai_provider->\Drupal::service('ai_provider');

was incorrect. Hence, replaced with:

$provider_model = $config->get('provider_model');

aligns with the form's configuration.

Looks good to merge!

Thanks for spotting, getting merged

Automatically closed - issue fixed for 2 weeks with no activity.