Fix 404 link to SoapUI assertions test step page

[yada]

Broken link (404)

• URL: https://www.soapui.org/functional-testing/assertion-teststep.html
• HTTP status: 404
• Found on: https://microcks.io/documentation/references/artifacts/soapui-conventions/

What to do

• Update the docs to point to the current SoapUI documentation page for assertions/test steps.
• Confirm the replacement is relevant for the SoapUI conventions section.

Help needed

If you’re unsure which SoapUI page is the right replacement, ask in comments and we’ll help pick one.