Changed language in Link Object to ensure either OperationRef or OperationId is present

Author: darrelmiller

versions/3.0.md

@@ -1971,7 +1971,7 @@ The presence of a link does not guarantee the caller's ability to successfully i
 
 Unlike _dynamic_ links (i.e. links provided **in** the response payload), the OAS linking mechanism does not require link information in the runtime response.
 
-For computing links, and providing instructions to execute them, a [runtime expression](#runtimeExpression) is used for accessing values in an operation and using them as parameters while invoking the linked operation.
+For computing links, and providing instructions to execute them, a [runtime expression](#runtimeExpression) is used for accessing values in an operation and using them as parameters while invoking the linked operation.  
 
 ##### Fixed Fields
 
@@ -1986,7 +1986,7 @@ Field Name  |  Type  | Description
 
 This object can be extended with [Specification Extensions](#specificationExtensions). 
 
-Locating a linked resource MAY be performed by either a `operationRef` or `operationId`.
+A linked operation MUST be identified using either an `operationRef` or `operationId`.
 In the case of an `operationId`, it MUST be unique and resolved in the scope of the OAS document.
 Because of the potential for name clashes, the `operationRef` syntax is preferred 
 for specifications with external references.