docs: make description more accurate

Author: kenjis

user_guide_src/source/tutorial/static_pages.rst

@@ -159,7 +159,7 @@ If the requested page doesn't exist, a "404 Page not found" error is shown.
 The first line in this method checks whether the page actually exists.
 PHP's native ``is_file()`` function is used to check whether the file
 is where it's expected to be. The ``PageNotFoundException`` is a CodeIgniter
-exception that causes the default error page to show.
+exception that causes the 404 Page Not Found error page to show.
 
 In the header template, the ``$title`` variable was used to customize the
 page title. The value of title is defined in this method, but instead of