Change docstring to correctly specify the support for Categorical

[Daniel-Nichol]

The docstring for Categorical specifies the support as 1,2,...|p|, however this is incorrect and the actual support is 0,1,...,|p|-1. Note that

import pymc3 as pm
data = [1,2,3,4,5,6]
with pm.Model() as die_model:
    p = pm.Dirichlet('p', np.ones(6))
    d = pm.Categorical('d', p=p, observed=data)

Gives the error "index 6 is out of bounds for size 6", whereas

data = [0,1,2,3,4,5]
with pm.Model() as die_model:
    p = pm.Dirichlet('p', np.ones(6))
    d = pm.Categorical('d', p=p, observed=data)

works as intended.

[twiecki]

Thanks!