code-style: correct section 1 heading

Section 1 doesn't contain file naming rule, so its heading shall
be just "Code layout".

refs #2119

Change-Id: I204bdcd8443674b2fed7a8f4fd0345bdfa1ea47a
diff --git a/docs/code-style.rst b/docs/code-style.rst
index 0f1897a..58a8fb0 100644
--- a/docs/code-style.rst
+++ b/docs/code-style.rst
@@ -9,8 +9,8 @@
     * NDN Platform "C++, C, C#, Java and JavaScript Code Guidelines".
       The original document available at `<http://named-data.net/codebase/platform/documentation/ndn-platform-development-guidelines/cpp-code-guidelines/>`_
 
-1. Code layout and file naming
-------------------------------
+1. Code layout
+--------------
 
 1.1. The code layout should generally follow the GNU coding standard layout for C,
 extended it to C++.