CodingGuidelines: move period inside parentheses

The contents within parenthesis should be omittable without resulting
in broken text.

Eliding the parenthesis left a period to end a run without any content.

Signed-off-by: Josh Soref <jsoref@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -450,7 +450,7 @@ For C programs:
    one of the approved headers that includes it first for you.  (The
    approved headers currently include "builtin.h",
    "t/helper/test-tool.h", "xdiff/xinclude.h", or
-   "reftable/system.h").  You do not have to include more than one of
+   "reftable/system.h".)  You do not have to include more than one of
    these.
 
  - A C file must directly include the header files that declare the