Fix getGPTCommand method to include targetLocale in extend call

[Zazama]

The README.md has a passage about extending the prompt dynamically, but the $locale parameter is not passed to the extension call.

public function updateGptCommand(&$command, $locale)
{
    $command = 'You are a professional translator. Translate the following text to ' . $locale . ' language. Please keep the json format intact.';
}

[adiwidjaja]

We moved the autotranslate functionality here: https://github.com/s2hub/silverstripe-autotranslate and included this functionality.