[no ci] Add type bound on MHContext rng

Author: penelopeysm

src/mcmc/mh.jl

@@ -406,7 +406,7 @@ function AbstractMCMC.step(
     return Transition(model, new_state.varinfo, nothing), new_state
 end
 
-struct MHContext{R} <: DynamicPPL.AbstractContext
+struct MHContext{R<:AbstractRNG} <: DynamicPPL.AbstractContext
     rng::R
 end
 DynamicPPL.NodeTrait(::MHContext) = DynamicPPL.IsLeaf()