Format README.md

What does this MR do?

Formats README.md to fix Markdown breaking.

• before: https://gitlab.com/gitlab-org/security-products/dast/-/blob/6fb9d36d5cac3c181397e23779369120f7fb8f9c/README.md
• after: https://gitlab.com/gitlab-org/security-products/dast/-/blob/9a3c3842f829d0b85ab1c18237dee2a7d6a06f1c/README.md

Does this MR meet the acceptance criteria?

• [n/a] Changelog entry added
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• [n/a] Tests added for this feature/bug
• [n/a] Job definition updated, if necessary
  • [n/a] Job definition example
  • [n/a] Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• [n/a] Conforms to the Go guidelines
• [n/a] Security reports checked/validated by reviewer