fix: index number in api/README (#3214)

api/README.md

    ```bash
    sed -i "/^SECRET_KEY=/c\SECRET_KEY=$(openssl rand -base64 42)" .env
    ```
-3.5 If you use Anaconda, create a new environment and activate it
+4. If you use Anaconda, create a new environment and activate it
    ```bash
    conda create --name dify python=3.10
    conda activate dify
    ```
-4. Install dependencies
+5. Install dependencies
    ```bash
    pip install -r requirements.txt
    ```
-5. Run migrate
+6. Run migrate
 
    Before the first launch, migrate the database to the latest version.
 
    pip install -r requirements.txt --upgrade --force-reinstall
    ```
 
-6. Start backend:
+7. Start backend:
    ```bash
    flask run --host 0.0.0.0 --port=5001 --debug
    ```
-7. Setup your application by visiting http://localhost:5001/console/api/setup or other apis...
-8. If you need to debug local async processing, you can run `celery -A app.celery worker -P gevent -c 1 --loglevel INFO -Q dataset,generation,mail`, celery can do dataset importing and other async tasks.
+8. Setup your application by visiting http://localhost:5001/console/api/setup or other apis...
+9. If you need to debug local async processing, please start the worker service by running 
+`celery -A app.celery worker -P gevent -c 1 --loglevel INFO -Q dataset,generation,mail`.
+The started celery app handles the async tasks, e.g. dataset importing and documents indexing.