Documentation/CodingStyle: fix example macro parenthesis imbalance

Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Fixes: f2027543 ('documentation: update CodingStyle on local variables naming in macros')
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>