Update API docs

(cherry picked from commit 99afb4a)

Author: deliahu

docs/applications/resources/apis.md

@@ -7,7 +7,7 @@ Serve models at scale and use them to build smarter applications.
 ```yaml
 - kind: api
   name: <string>  # API name (required)
-  model: <string>  # reference to a Cortex model (this or external_model must be specified)
+  model: <string>  # reference to a model (e.g. @dnn) (this or external_model must be specified)
   external_model:  # (this or model must be specified)
     path: <string>  # path to a zipped model dir (e.g. s3://my-bucket/model.zip)
     region: <string>  # S3 region (default: us-west-2)