docs: update info for testing, data extract not required

docs/testing-visually.md

@@ -8,18 +8,16 @@ If you wish to view the split data output in a more visual way:
 
 1. Generate a geojson bbox via [geojson.io](https://geojson.io)
 2. Copy the content into `output/input.geojson` in this repo.
-3. Create a data extract and place inside `output/extract.geojson`.
-4. Run the splitting algorithm:
+3. Run the splitting algorithm:
 
    ```bash
    docker compose run --rm splitter fmtm-splitter \
        --boundary output/input.geojson \
-       --extract output/extract.geojson \
        --outfile output/output.geojson \
        --number 50
    ```
 
-5. Copy the data from `output/output.geojson` to geojson.io to visualise.
+4. Copy the data from `output/output.geojson` to geojson.io to visualise.
 
 ## Option 2: FMTM