Use official SPDX ID and correct version number
Use official SPDX ID and correct version number

https://spdx.org/licenses/

..additionally, this code is licensed under MPL-2.0; I'm not sure there /is/ a 3.0?

This change /may/ warrant a minor version bump; if so LMK and I'll update the PR.
hashicorp/vault-rails,hashicorp/vault-rails