security: Add doxygen comments

Change-Id: Ieafe4d13513e626d84ef9e621694f69844785018
diff --git a/src/security/public-key.cpp b/src/security/public-key.cpp
index 09222a4..c8362af 100644
--- a/src/security/public-key.cpp
+++ b/src/security/public-key.cpp
@@ -115,4 +115,4 @@
   return os;
 }
 
-}
+} // namespace ndn