Fix to_submodel entry

Author: penelopeysm

main.jl

@@ -33,7 +33,7 @@ macro register(model)
 end
 
 # These imports tend to get used a lot in models
-using DynamicPPL: @model
+using DynamicPPL: @model, to_submodel
 using Distributions
 using LinearAlgebra: I