Switch the Lambda function from PUT to POST

This is needed because I thought I had done it earlier (when I committed the change to the Drupal REST endpoint: https://github.com/geerlingguy/drupal-rekognition_api/commit/4c2d6c84d24...), but I didn't.

So currently, if you use the included Lambda function with the included endpoint, you get something like:

Symfony\Component\HttpKernel\Exception\MethodNotAllowedHttpException: No route found for "PUT /rekognition_api/objects/create": Method Not Allowed (Allow: POST) in Symfony\Component\HttpKernel\EventListener\RouterListener->onKernelRequest() (line 180 of /var/www/drupal/vendor/symfony/http-kernel/EventListener/RouterListener.php).