Fix substitution signature (#242)

Author: blegat

src/substitution.jl

@@ -3,7 +3,7 @@ export subs
 
 # TODO Vararg{<:...} -> Vararg{...}
 const Substitution = Pair{<:AbstractVariable}
-const MultiSubstitution{N} = Pair{<:Tuple{Vararg{AbstractVariable, N}}, <:Tuple{Vararg{<:Any, N}}}
+const MultiSubstitution{N} = Pair{<:Tuple{Vararg{AbstractVariable, N}}, <:Tuple{Vararg{Any, N}}}
 const MultiVectorSubstitution = Pair{<:Tuple{Vararg{AbstractVariable}}, <:AbstractVector}
 
 # When the variables are promoted to be in the same vector they could be promoted into a monomial