Fix FAQ sample link

[JSap0914]

Summary

Fixes the FAQ sample test link that currently points to a non-existent app68 directory.

The sample exists under the current v31 test path in springdoc-openapi, so this updates the AsciiDoc link target and visible URL to /api/v31/app68.

Fixes #106.

Evidence

• Issue: dead link in faq #106
• Duplicate PR check before edits: gh pr list -R springdoc/springdoc.github.io --state open --search '106 OR #106 OR faq.adoc OR app68 OR springdoc-openapi-actuator-webmvc-tests' --json number,title,url,author,headRefName,baseRefName,updatedAt returned [].
• Current broken URL returned HTTP_STATUS=404 before the edit.
• Updated URL returned HTTP_STATUS=200 after the edit.

Verification

• curl -L -o /dev/null -s -w '%{http_code}' https://github.com/springdoc/springdoc-openapi/tree/main/springdoc-openapi-tests/springdoc-openapi-actuator-webmvc-tests/src/test/java/test/org/springdoc/api/v31/app68 -> 200
• git diff --check -> passed

Full docs generation was not run in this environment because mvn is not installed, there is no mvnw wrapper in the repository, and asciidoctor is not installed.

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