The docs ask for SYMFONY__DATABASE_URL, but the code asks for DATABASE_URL

Here: https://github.com/dunglas/api-platform-heroku

You say:
```
In your local development environment, don't forget to set the SYMFONY__DATABASE_URL environment variable. A typical value for that variable is `postgres://user:pass@server:5432/dbname.
```

But in the code here: https://github.com/dunglas/api-platform-heroku/blob/6e6f68764c31e679033c82acd85430d7b4b6d482/src/Database.php#L26

You are looking for 'DATABASE_URL'

Compare the documentation passage linked in the issue with src/Database.php at line 26, which reads the DATABASE_URL environment variable. Update the documented variable name so it matches the code, then check the surrounding documentation for the same reference. Done means the local-development instructions and the implementation consistently use one variable name.

Written by the indexing model from the issue text.