update verbiage

Co-authored-by: Pierre-Anthony Lemieux <pal@sandflow.com>

Author: SteveLLamb

doc/main.html

@@ -569,7 +569,7 @@ <h4>Normative references section</h4>
         </ul>
           <p>Undated references should be used, since they point to the most most up-to-date edition of a document.</p>
 
-          <p>Dated references may be used when referencing a specific element of the document is necessary, e.g., a table, or when past and future editions of the document are likely to be inadequate. </p>
+          <p>Dated references shall be used when referencing a specific element of the document, e.g., a table. They should also be used when past and future editions of the document are likely to be inadequate. </p>
 
           <p class="note">DOI links should be used for both the most recent and specific editions of a SMPTE document. SMPTE HO can provide guidance for specific DOIs available.</p>