docs: simplify conf.py and fix compatibility with upcoming Sphinx 6.0

Change-Id: Iacf4a72d30babc4a7c3fab0fea619a798eff2df1
diff --git a/daemon/nfd.cpp b/daemon/nfd.cpp
index a178fe4..5cda79e 100644
--- a/daemon/nfd.cpp
+++ b/daemon/nfd.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2014-2021,  Regents of the University of California,
+ * Copyright (c) 2014-2022,  Regents of the University of California,
  *                           Arizona Board of Regents,
  *                           Colorado State University,
  *                           University Pierre & Marie Curie, Sorbonne University,
@@ -45,7 +45,7 @@
 
 NFD_LOG_INIT(Nfd);
 
-const std::string INTERNAL_CONFIG("internal://nfd.conf");
+const std::string INTERNAL_CONFIG{"internal://nfd.conf"};
 
 Nfd::Nfd(ndn::KeyChain& keyChain)
   : m_keyChain(keyChain)