PrimaryPackagePurpose field output OPERATING_SYSTEM instead of official value OPERATING-SYSTEM

[loulou123546]

In SPDX v2.3, Package can have a property "PrimaryPackagePurpose" which support a defined set of value.

SPDX reference use "OPERATING-SYSTEM" value while the spdx_tools library output "OPERATING_SYSTEM" with underscore instead of dash. This may cause issue with ingestion of SPDX by other tools considered as invalid.

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