add-autogenerated-comment causes an error

[JustinFullerNYT]

I am using this from the README:

-add-autogenerated-comment to add comment on top of the generated protos that those files are autogenerated and should not be modified. This is disabled by default.

My command:

openapi2proto -add-autogenerated-comment -spec ./service/api.yaml -out ./service/api.proto

Output:

flag provided but not defined: -add-autogenerated-comment