Outdated code sample in Mixture documentation

[BatyLeo]

This code sample seems outdated in the mixture page of the documentation.

Instead, according to the mixturemodel.jl source file it should be:

abstract type AbstractMixtureModel{VF<:VariateForm,VS<:ValueSupport,C<:Distribution} <: Distribution{VF, VS} end

struct MixtureModel{VF<:VariateForm,VS<:ValueSupport,C<:Distribution,CT<:Categorical} <: AbstractMixtureModel{VF,VS,C}
    components::Vector{C}
    prior::CT
end

const UnivariateMixture{S<:ValueSupport,   C<:Distribution} = AbstractMixtureModel{Univariate,S,C}
const MultivariateMixture{S<:ValueSupport, C<:Distribution} = AbstractMixtureModel{Multivariate,S,C}

The additional C<:Distribution is needed in order to be able to define a custom mixture without error.