Point the reference at the OpenAPI spec

The previous reference URL 404s. There is no rendered docs page; the
spec at zillapi.com/openapi.json is the canonical reference and is the
same document the skill tells the reader to verify endpoints against.

Author: zillapi

skills/us-property-data/SKILL.md

@@ -62,4 +62,5 @@ Ask for one real address end to end before trusting generated code. A property l
 
 ## Reference
 
-API documentation: https://zillapi.com/docs
+OpenAPI specification (canonical, machine-readable): https://zillapi.com/openapi.json
+Site: https://zillapi.com/