message: fix minor doc inaccuracy
diff: #Working with OpenAPI Specifications
  -> NOTE: OpenAPI is the official name of the specification that was formally called "Swagger".
  +> NOTE: OpenAPI is the official name of the specification that was formerly called "Swagger".
  AutoRest processes OpenAPI specifications following the [OpenAPI RESTful API Documentation Specification](Swagger-spec2.0)
  API descriptions that are valid according to the schema can produce client libraries that are not very user-friendly. Here are some techniques to apply in authoring OpenAPI that improve the usability of the client generated by AutoRest.
  