Fix dirichlet type

[deoxyribose]

Before

using Gen

@gen function rand_prob_vec()
    prob_vec ~ dirichlet([1.0 for _ in 1:3])
end

tr = simulate(rand_prob_vec, ())

throws

ERROR: MethodError: Cannot `convert` an object of type Vector{Float64} to an object of type Float64

Closest candidates are:
  convert(::Type{T}, ::DualNumbers.Dual) where T<:Union{Real, Complex}
   @ DualNumbers ~/.julia/packages/DualNumbers/5knFX/src/dual.jl:24
  convert(::Type{R}, ::T) where {R<:Real, T<:ReverseDiff.TrackedReal}
   @ ReverseDiff ~/.julia/packages/ReverseDiff/UJhiD/src/tracked.jl:260
  convert(::Type{T}, ::T) where T
   @ Base Base.jl:84
  ...

Stacktrace:
 [1] traceat(state::Gen.GFSimulateState, dist::Gen.Dirichlet, args::Tuple{Vector{Float64}}, key::Symbol)
   @ Gen ~/.julia/packages/Gen/ME5el/src/dynamic/simulate.jl:19
 [2] var"##rand_prob_vec#225"(state#227::Gen.GFSimulateState)
   @ Main ~/synthesis/frans/test_dirichlet.jl:4
 [3] exec(gen_fn::DynamicDSLFunction{Any}, state::Gen.GFSimulateState, args::Tuple{})
   @ Gen ~/.julia/packages/Gen/ME5el/src/dynamic/dynamic.jl:58
 [4] simulate(gen_fn::DynamicDSLFunction{Any}, args::Tuple{})
   @ Gen ~/.julia/packages/Gen/ME5el/src/dynamic/simulate.jl:61

[ztangent]

Thanks for noticing this, surprised our checks didn't catch it! While you're at it, do you think you could adjust the docstring so it fits the rest of the distributions? Currently it is:

"""
    Dirichlet(alpha::Vector{Float64})

Sample a simplex Vector{Float64} from a Dirichlet distribution.
"""

But it shouldn't be capitalized. It should be:

"""
    dirichlet(alpha::Vector{Float64})

Sample a simplex Vector{Float64} from a Dirichlet distribution.
"""

[ztangent]

Awesome, merged!