fix: clarify upload-sarify category uniqueness error message

[crenshaw-dev]

I spent quite a while wondering why my category input didn't seem to be working. Eventually had to fix it by editing the sarif file. https://github.com/argoproj/argo-cd/pull/10171/files

Merge / deployment checklist

• [ ] Confirm this change is backwards compatible with existing workflows.
• [ ] Confirm the readme has been updated if necessary.
• [ ] Confirm the changelog has been updated if necessary.