doc: Fix statement of RCU's memory-ordering requirements

The sentence defining the relationship of accesses before a grace
period to read-side accesses following that same grace period was
missing a small word: "not".  This commit therefore adds it.

Reported-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>