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-0.6.6.rst b/docs/release-notes/release-notes-0.6.6.rst
index e09f916..858f44b 100644
--- a/docs/release-notes/release-notes-0.6.6.rst
+++ b/docs/release-notes/release-notes-0.6.6.rst
@@ -23,7 +23,8 @@
 - ``NDN_THROW`` macro to throw exceptions including file position and runtime context of the
   throw statement (:issue:`4834`)
 
-- Ensure that a ``Block`` with TLV-TYPE zero is treated as invalid (:issue:`4726`, :issue:`4895`)
+- Ensure that a :ndn-cxx:`Block` with TLV-TYPE zero is treated as invalid (:issue:`4726`,
+  :issue:`4895`)
 
 Improvements and bug fixes
 ^^^^^^^^^^^^^^^^^^^^^^^^^^