docs/FLY.md links to a README.md anchor that does not exist

## What

docs/FLY.md links to a README.md section that does not exist.

## Where

docs/FLY.md:57:

```
See the [README](../README.md#configuration) for the list.
```

README.md has no heading that produces the anchor `#configuration`. The section actually being pointed at is the "### Environment Variables" heading, whose anchor is `#environment-variables`.

## Expected

The link resolves to the Environment Variables section.

## How to fix

Change the anchor in docs/FLY.md:57 from `#configuration` to `#environment-variables`.

## How to check

Open the rendered page and confirm the link lands on "Environment Variables" in README.md.