docs: Match the environment variable with upstream

[pan93412]

FrankenWP reuses the upstream docker-entrypoint.sh, and this script requires a WORDPRESS_ variable before it copies wp-config.php from the template.

Besides, WORDPRESS_* variables are also worked on this image.

https://github.com/docker-library/wordpress/blob/0015d465b4115ade0e0f98b3df8b5c17ec4a98e4/docker-entrypoint.sh#L70-L71