Fix a code typo in the README file

[sangaline]

The README currently lists an example of

    import numpy as np
    from ionmf.factorization.onmf import onmf
    X = np.random.rand(10, 10)
    W, H = onmf(X, k=5, alpha=1.0)

but this code doesn't work because k isn't a valid argument to onmf. This PR replaces k with rank which is the actual parameter for the number of dimensions.