Fix solo-kit's hack/update-codegen.sh template

Right now the hack/update-codegen.sh bash script was broken in https://github.com/solo-io/solo-kit/commit/f03f85987f45d1118b317e855f5d5e16a9ebbae7. I took an initial stab at fixing this in https://github.com/solo-io/solo-kit/pull/564 but I don't have the capacity to push that PR though right now.