docs/memory-barriers.txt: Fix a typo in CPU MEMORY BARRIERS section

Commit 39323c64 ("smp_mb__{before,after}_atomic(): update Documentation")
has a typo in CPU MEORY BARRIERS section:
"RMW functions that do not imply are memory barrier are ..." should be
"RMW functions that do not imply a memory barrier are ...".

This patch fixes this typo.

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>