docs/memory-barriers.txt: Add a missed closing parenthesis

The description of io_stop_wc(), which was added by commit d5624bb2
("asm-generic: introduce io_stop_wc() and add implementation for ARM64"),
has an unclosed parenthesis.  This commit closes it.

Fixes: d5624bb2 ("asm-generic: introduce io_stop_wc() and add implementation for ARM64")
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>