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/docs/code-style.rst b/docs/code-style.rst
index 48984bb..dfad354 100644
--- a/docs/code-style.rst
+++ b/docs/code-style.rst
@@ -748,7 +748,7 @@
 
     .. code-block:: c++
 
-        #include "ndn-cxx/detail/pending-interest.hpp"
+        #include "ndn-cxx/impl/pending-interest.hpp"
         #include "ndn-cxx/util/random.hpp"
 
         #include <cstdlib>