impl: rename 'detail' to 'impl'

'impl' directory contains implementation-detail headers that are NOT
installed. 'detail' directory will be used for implementation-detail
headers that ARE installed.

refs #4782

Change-Id: I13428e032c93207753a6b856c693511c4fc3bbb8
diff --git a/tests/integrated/network-monitor.cpp b/tests/integrated/network-monitor.cpp
index 4c4467b..57cea97 100644
--- a/tests/integrated/network-monitor.cpp
+++ b/tests/integrated/network-monitor.cpp
@@ -26,7 +26,7 @@
 
 #include "ndn-cxx/net/network-address.hpp"
 #include "ndn-cxx/net/network-interface.hpp"
-#include "ndn-cxx/net/detail/link-type-helper.hpp"
+#include "ndn-cxx/net/impl/link-type-helper.hpp"
 #include "ndn-cxx/util/string-helper.hpp"
 #include "ndn-cxx/util/time.hpp"