util: add missing include for `std::set`

Change-Id: If18b2f6196e01949b380b41b42ae09e460310ba5
diff --git a/docs/conf.py b/docs/conf.py
index 4d8c5f4..fc4ee3a 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -10,7 +10,7 @@
 # https://www.sphinx-doc.org/en/master/usage/configuration.html#project-information
 
 project = 'ndn-cxx: NDN C++ library with eXperimental eXtensions'
-copyright = 'Copyright © 2013-2022 Regents of the University of California.'
+copyright = 'Copyright © 2013-2023 Regents of the University of California.'
 author = 'Named Data Networking Project'
 
 # The short X.Y version.