Add SPDX format to sbomnix description

[ktusawrk]

In https://tiiuae.github.io/ghaf/scs/sbom.html#sbom-tooling-in-ghaf paragraph there is text

Initially, sbomnix will support CycloneDX SBOM specification, due to the availability of other open source tools that also support CycloneDX. Support for other SBOM formats to sbomnix might be added in later versions.

Because SPDX specification support has been added to sbomnix, the paragraph should say

sbomnix supports CycloneDX and SPDX SBOM specifications.

[brianmcgillion]

https://tiiuae.github.io/ghaf/scs/sbom.html?highlight=spdx#sbom-tooling-in-ghaf