[issue-827] fix mispelling of OPERATING-SYSTEM

[loulou123546]

A fix for issue #827

SPDX 2.3 reference use the term "OPERATING-SYSTEM" instead of "OPERATING_SYSTEM" which is was is output by this library.

Documentation: https://spdx.github.io/spdx-spec/v2.3/package-information/#724-primary-package-purpose-field

[loulou123546]

Hello, can someone do a review of it?

[armintaenzertng]

Hi @loulou123546, thanks for your contribution! This has always been ambiguous as the json schema did not match the specification you cite. However, by now, the json schema allows for both dashes and underscores, so we could indeed switch to the more spec-conform dashes. There is also the referenceCategory which has the same problem (see here in our code). We should change that to use dashes there, too, so that we stay consistent, I think. Do you want to take care of that, too? I have to look into why the pipeline does not start for this PR...