Update .env.example to reflect latest version of vlucas/phpdotenv.

[mpdonadio]

The latest version of vlucas/phpdotenv doesn't support getenv() in Dotenv::createImmutable(); you need to use one of the superglobals. This updates .env.example to reflect proper usage.

[weitzman]

Thats strange. This PR is a regression at least in older php versions. See https://stackoverflow.com/a/21473853/265501

[mpdonadio]

If we are targeting the drupal-9.0.x branch, then the composer.json has a min of PHP 7.3, and drupal-drupal-8.x.y is 7.0.8, so I am not sure we need to worry too much about old PHP versions.

From https://github.com/vlucas/phpdotenv/blob/master/README.md#putenv-and-getenv

Using getenv() and putenv() is strongly discouraged due to the fact that these functions are not thread safe, however it is still possible to instruct PHP dotenv to use these functions. Instead of calling Dotenv::createImmutable, one can call Dotenv::createUnsafeImmutable, which will add the PutenvAdapter behind the scenes. Your environment variables will now be available using the getenv method, as well as the super-globals:

load.environment.php uses Dotenv::createImmutable(), so I think the doc fix is the safest.

Here is a mini example of the problem, https://github.com/mpdonadio/phpdotenv-example