Add missing DTLSv1.2 reference

See https://mailarchive.ietf.org/arch/msg/core/LPRIaSYqNKMmEPJqCqySmceXP7s/

draft-ietf-core-dns-over-coap.md

@@ -47,6 +47,7 @@ author:
 
 normative:
  RFC1035: dns
+ RFC6347: dtls12
  RFC7228: constr-nodes
  RFC7252: coap
  RFC7641: coap-observe
@@ -94,7 +95,7 @@ Introduction
 This document defines DNS over CoAP (DoC), a protocol to send DNS
 {{-dns}} queries and get DNS responses over the Constrained Application
 Protocol (CoAP) {{-coap}}. Each DNS query-response pair is mapped into a
-CoAP message exchange. Each CoAP message is secured by DTLS {{-dtls13}} or
+CoAP message exchange. Each CoAP message is secured by DTLS {{-dtls12}} {{-dtls13}} or
 Object Security for Constrained RESTful Environments (OSCORE) {{-oscore}}
 to ensure message integrity and confidentiality.