fixup SemEnsemble (minor)

Author: Alexey Stukalov

src/frontend/specification/Sem.jl

@@ -262,7 +262,7 @@ function sem_term(model::AbstractSem, id::Any)
     return term
 end
 
-function sem_term(model::AbstractSem, id::Nothing = nothing)
+function sem_term(model::AbstractSem, _::Nothing = nothing)
     if nsem_terms(model) != 1
         error(
             "Model contains $(nsem_terms(model)) SEM terms, you have to specify a specific term",