Fix wording in DAST API heading at CONTRIBUTING.md

What does this MR do?

Fix wording in DAST API heading at CONTRIBUTING.md: "DAST Analyzer API" looks not in line with our terminology "DAST scanner API" seems to be more appropriate.

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
  • [-] Job definition example
  • [-] Vendored CI Templates (also in CE)
• [-] Conforms to the code review guidelines
• [-] Conforms to the Go guidelines
• [-] Security reports checked/validated by reviewer