Update callback signature in docs (#130)

Author: devmotion

docs/src/api.md

@@ -71,8 +71,8 @@ Common keyword arguments for regular and parallel sampling are:
 - `progress` (default: `AbstractMCMC.PROGRESS[]` which is `true` initially):  toggles progress logging
 - `chain_type` (default: `Any`): determines the type of the returned chain
 - `callback` (default: `nothing`): if `callback !== nothing`, then
-  `callback(rng, model, sampler, sample, iteration)` is called after every sampling step,
-  where `sample` is the most recent sample of the Markov chain and `iteration` is the current iteration
+  `callback(rng, model, sampler, sample, state, iteration)` is called after every sampling step,
+  where `sample` is the most recent sample of the Markov chain and `state` and `iteration` are the current state and iteration of the sampler
 - `discard_initial` (default: `0`): number of initial samples that are discarded
 - `thinning` (default: `1`): factor by which to thin samples.