Small Typo at the Latex part of param_mixture.py

[cruyffturn]

I think there is a little typo which gives the following error: TypeError: init() got an unexpected keyword argument 'mu'

I checked the "mixture_gaussian_gibbs.py" example and I suggest the following suggestion params = {'loc': tf.zeros(5), 'scale': tf.ones(5)}

https://github.com/blei-lab/edward/blob/352596c3bc77e1cb5c01f77ac3283a53c74dbf21/edward/models/param_mixture.py#L36

Thanks for this wonderful library.