Update outdated instructions in scripts/generate_pip_deps_from_conda.py
It says
```
Usage:

    Generate `requirements-dev.txt`
    $ ./conda_to_pip

    Compare and fail (exit status != 0) if `requirements-dev.txt` has not been
    generated with this script:
    $ ./conda_to_pip --compare
```
but it should be
```
Usage:

    Generate `requirements-dev.txt`
    $ python scripts/generate_pip_deps_from_conda.py

    Compare and fail (exit status != 0) if `requirements-dev.txt` has not been
    generated with this script:
    $ python scripts/generate_pip_deps_from_conda.py --compare
```

No need to ask if you can work on this, feel free to just submit a PR