Demo/API example: use /api prefix with UI port

Author: NikolayS

engine/api/swagger-spec/dblab_server_swagger.yaml

@@ -21,7 +21,7 @@ externalDocs:
   url: https://gitlab.com/postgres-ai/docs/tree/master/docs/database-lab
 
 servers:
-  - url: "https://demo.aws.postgres.ai:446"
+  - url: "https://demo.aws.postgres.ai/api"
     description: "DBLab 3.x demo server; token: 'demo-token'"
     x-examples:
       Verification-Token: "demo-token"