Missing step to uncomment dotenv configuration

We should mention somewhere around [here](https://www.apollographql.com/docs/tutorial/production.html#api-key) that [this line](https://github.com/apollographql/fullstack-tutorial/blob/master/final/server/src/index.js#L1) should be uncommented in order for `dotenv` configuration to take effect. Without this, metrics are not sent to Apollo Engine since the Apollo server does not actually have the ENGINE_API_KEY set. Also, some acknowledgement of how this works via the `dotenv` module seems like it would be appropriate.