Reference default dir for DS_REPORT_URL

What does this MR do?

This MR updates the DS_REPORT_URL to point to the default subdir, since the location of the expectation has changed. See this discussion for more details.

This can only be merged after Move expectation files (gitlab-org/security-products/analyzers/gemnasium-python!166 - merged)

What are the relevant issue numbers?

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer