fix: fix reference to doc.rfc_number in template

Author: jennifer-richards

ietf/templates/doc/document_html.html

@@ -10,7 +10,7 @@
           href="/feed/document-changes/{{ doc.name }}/">
     <meta name="description"
           {% if doc.get_state_slug == "rfc" %}
-              content="{{ doc.title }} (RFC {{ rfc_number }}{% if published %}, {{ published.time|date:"F Y" }}{% endif %}{% if obsoleted_by %}; obsoleted by {{ obsoleted_by|join:", " }}{% endif %})"
+              content="{{ doc.title }} (RFC {{ doc.rfc_number }}{% if published %}, {{ published.time|date:"F Y" }}{% endif %}{% if obsoleted_by %}; obsoleted by {{ obsoleted_by|join:", " }}{% endif %})"
           {% else %}
               content="{{ doc.title }} (Internet-Draft, {{ doc.time|date:"Y" }})"
           {% endif %}>