Apply improved wording around status codes from 3.1.1 (original PR OAI#2630)

Author: lornajane

versions/3.0.4.md

@@ -94,7 +94,7 @@ Some examples of possible media type definitions:
 ```
 ##### <a name="httpCodes"></a>HTTP Status Codes
 The HTTP Status Codes are used to indicate the status of the executed operation. 
-The available status codes are defined by [RFC7231](https://tools.ietf.org/html/rfc7231#section-6) and registered status codes are listed in the [IANA Status Code Registry](https://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml).
+Status codes SHOULD be selected from the available status codes registered in the [IANA Status Code Registry](https://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml).
 
 ## Specification