CONTRIBUTING: Update reference to kernel coding style

Kernel coding style guide filename is stale, this file has been renamed
in the kernel tree.  While this file still exists we should use the new
filename.

Update reference to kernel coding style guide to use the new file name.

Signed-off-by: Tobin C. Harding <me@tobin.cc>

CONTRIBUTING

@@ -14,8 +14,9 @@ Coding Style:
 
 The coding style follows the Linux kernel coding style.
 
-The coding style is defined in the 'CodingStyle' file, available in
-the directory 'Documentation' of the Linux kernel source tree.
+The Linux kernel coding style guide can be found within the kernel tree:
+
+	Documentation/process/coding-style.rst
 
 It can be accessed online too: