doc: rcu: update printed dynticks counter bits

[ Upstream commit 4a09e358 ]

The stall warning prints 16 bits since commit 17147677
("context_tracking: Convert state to atomic_t").

Fixes: 17147677 ("context_tracking: Convert state to atomic_t")
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>