step_arg definition in Alethe specification's proof grammar is outdated

Page 9 of the specification says ⟨step_arg⟩ ≔ ⟨symbol⟩ | ( ⟨symbol⟩ ⟨proof_term⟩ ), but this doesn't seem to be how step arguments are currently being supplied in practice. Correct me if I'm wrong, but I think a ⟨step_arg⟩ is just a ⟨proof_term⟩ now.