chore: Update environment variable seperator comment to indicate double underscore

[bgins]

Description

This PR makes the following changes:

• Update environment variable separator to one underscore It should be a double underscore!
• Update comment to indicate a double underscore should be used

Type of change

• Documentation

Test plan (required)

No test, check the updated comment.

[QuinnWilton]

This LGTM! @zeeshanlakhani was there a reason you'd done two underscores originally? I'd noticed that happening in the fission-server too, and wasn't sure whether that was an intentional choice.

[bgins]

@QuinnWilton was just chatting with @zeeshanlakhani. Turns out it should be double underscores, the comment is wrong. 🙃

Updating the PR to fix the comment.

[zeeshanlakhani]

This LGTM! @zeeshanlakhani was there a reason you'd done two underscores originally? I'd noticed that happening in the fission-server too, and wasn't sure whether that was an intentional choice.

@QuinnWilton, after talking to @bgins, it should be two underscores, but the comment is wrong :).

Example (overriding toml configuration):

[server]
environment = "local"
metrics_port = 4000
port = 3030

To override environment, you'd use APP__SERVER__ENVIRONMENT="dev". Why 2 > 1? Well, metrics port would be APP__SERVER__METRICS_PORT=2222