Update samtranslator/swagger/swagger.py

Co-authored-by: GZ <yuanhaoz@amazon.com>

samtranslator/swagger/swagger.py

@@ -65,7 +65,7 @@ def __init__(self, doc: Optional[Dict[str, Any]]) -> None:
             raise InvalidDocumentException(
                 [
                     InvalidTemplateException(
-                        "Invalid Swagger document. May be caused by having the OpenApi definition defined outside of the template."
+                        "Invalid Swagger document or the Swagger document is not explicitly defined in `DefinitionBody`."
                     )
                 ]
             )