License tag should mention SPDX format

[pemensik]

Example at https://rpm-software-management.github.io/rpm/manual/spec.html still lists legacy License example with GPLv3. I think it might have switched not only on Fedora to SPDX, probably that should be given as example instead? It may confuse people.

Unless there is disagreement between distributions, I think SPDX should be recommended and given as example instead.

[pemensik]

Hmm, and is limit < 70 characters still valid? It seems to be a lot of packages in Rust could not fit there.