Documentation: coding-style: Clean up header guard style

Header guards were replaced by pragma once calls with commits
0a64cf8b76 ("libcamera: Convert to pragma once") through to
796210ecea ("v4l2: Convert to pragma once")

Update the coding style to reflect the updated header guard style.

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Stefan Klug <stefan.klug@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

Documentation/coding-style.rst

@@ -59,7 +59,7 @@ document:
   underscores in between
 * All formatting rules specified in the selected sections of the Linux kernel
   Code Style for indentation, braces, spacing, etc
-* Header guards are formatted as '__LIBCAMERA_FILE_NAME_H__'
+* Headers are guarded by the use of '#pragma once'
 
 Order of Includes
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