Incorrect use of `MONGODB_URL` to demonstrate the `%env(url:...)%` processor

[GromNaN]

In configuration/env_var_processors.rst, we need to find a different example than MONGODB_URL, which is incorrect.

MongoDB has a connection string (or MONGODB_URI) that can contain multiple host. Which is invalid for parse_url.

The example should use a real URL, like an API endpoint.