docs: fix running instructions

[joscha]

For a generation like this:

dlt-init-openapi affinity --url https://raw.githubusercontent.com/planet-a-ventures/affinity-node/refs/heads/main/openapi/2024-11-18.json --global-limit 2

the following scaffold is generated:

.
├── README.md
├── affinity
│   └── __init__.py
├── affinity_pipeline.py
├── requirements.txt
└── rest_api
    ├── README.md
    ├── __init__.py
    ├── config_setup.py
    ├── exceptions.py
    ├── requirements.txt
    ├── typing.py
    └── utils.py

inside ./affinity_pipeline/

which means that:

cd affinity_pipeline
python pipeline.py

yields:

/Users/joscha/dev/dlt-test/.devenv/state/venv/bin/python: can't open file '/Users/joscha/dev/dlt-test/affinity_pipeline/pipeline.py': [Errno 2] No such file or directory