mnf documentation examples use invalid call signature

In the following lines of the `spectral.mnf` doc string, `data` should be added as the first argument:

    >>> denoised = mnfr.denoise(snr=10)

    >>> # Reduce dimensionality, retaining NAPC components where SNR >= 10.
    >>> reduced = mnfr.reduce(snr=10)

    >>> # Reduce dimensionality, retaining top 50 NAPC components.
    >>> reduced = mnfr.reduce(num=50)
