Fix YAML syntax error

README.md

@@ -29,7 +29,8 @@ To set up deployment:
    - `DOMAIN`: domain name of the live site (e.g. `site.example.tld`)
    - `STACK_NAME`: the Docker stack name, usually `$DOMAIN` with dots replaced
        with underscores (e.g. `site_example_tld`)
-   `STACK_NAME` / `DOMAIN` of the deployed app.
+   - `DRONE_URL`: root URL for the Drone instance (e.g.
+       `https://drone.example.tld`)
 3. Make sure that DNS records for `$DOMAIN` and `admin.$DOMAIN`, pointing to
    `$HOST`, are defined.
 4. Generate secrets (`openssl rand -hex 32` works well) for `PAYLOAD_SECRET` and