Fix signature of isdone in docstring (#91)

* Fix signature of `isdone` in docstring

* Update sample.jl

Author: devmotion

src/sample.jl

@@ -44,9 +44,10 @@ convergence criterion `isdone` returns `true`, and return the samples.
 
 The function `isdone` has the signature
 ```julia
-isdone(rng, model, sampler, samples, iteration; kwargs...)
+isdone(rng, model, sampler, samples, state, iteration; kwargs...)
 ```
-and should return `true` when sampling should end, and `false` otherwise.
+where `state` and `iteration` are the current state and iteration of the sampler, respectively.
+It should return `true` when sampling should end, and `false` otherwise.
 """
 function StatsBase.sample(
     rng::Random.AbstractRNG,