docs: delete certificate spec

Moved to the NDN-packet-spec repo
https://github.com/named-data/NDN-packet-spec/commit/3a74e134979606af9a2b90b12f141f8794ad96d7

Change-Id: I57aec08aeb553d5eb6c5a7c368ba03b904b1e4ed
diff --git a/docs/release-notes/release-notes-0.5.0.rst b/docs/release-notes/release-notes-0.5.0.rst
index 914c5de..6c6a750 100644
--- a/docs/release-notes/release-notes-0.5.0.rst
+++ b/docs/release-notes/release-notes-0.5.0.rst
@@ -112,7 +112,8 @@
   ``ndn::CertificateExtension``, ``ndn::CertificateSubjectDescription``.  When necessary, use
   ``security::v1::PublicKey``, ``security::v1::Certificate``, ``security::v1::IdentityCertificate``,
   ``security::v1::CertificateExtension``, ``security::v1::CertificateSubjectDescription`` instead.
-  The next release will feature :doc:`a new version of the NDN Certificate format </specs/certificate>`.
+  The next release will feature `a new version of the NDN Certificate format
+  <https://named-data.net/doc/NDN-packet-spec/0.3/certificate.html>`__.
 
 Removed
 ^^^^^^^