update code sample for visibility levels

content/targets.rst

@@ -100,6 +100,21 @@ Why it is robust to use targets and properties than using variables? Given a tar
 	In this demo, we split the source code into 3 libraries and all files are available in the ``content/code/04_visibility-levels/`` folder.
 
 
+  .. code-block:: text
+
+  .
+  ├── CMakeLists.txt
+  ├── greeting
+  │   ├── greeting.cpp
+  │   └── greeting.hpp
+  ├── hello_world
+  │   ├── hello_world.cpp
+  │   └── hello_world.hpp
+  ├── main.cpp
+  └── world
+      ├── world.cpp
+      └── world.hpp
+
 
 In this source code, the main function links to greeting which links to hello_world which links to world.