Update psr4_class_loader.rst

Incorrect path shown in directory structure. demo.php refers to /lib/ but directory structure asks to add downloaded components to /libs/

Author: siva-epari

components/class_loader/psr4_class_loader.rst

@@ -27,7 +27,7 @@ The directory structure will look like this:
 
 .. code-block:: text
 
-    libs/
+    lib/
         ClassLoader/
             Psr4ClassLoader.php
             ...