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: NFox Chen <foxhlchen@gmail.com>
Acked-by: NWill Deacon <will@kernel.org>
Signed-off-by: NPaul E. McKenney <paulmck@kernel.org>

Documentation/memory-barriers.txt

@@ -1870,7 +1870,7 @@ There are some more advanced barrier functions:
 
      These are for use with atomic RMW functions that do not imply memory
      barriers, but where the code needs a memory barrier. Examples for atomic
-     RMW functions that do not imply are memory barrier are e.g. add,
+     RMW functions that do not imply a memory barrier are e.g. add,
      subtract, (failed) conditional operations, _relaxed functions,
      but not atomic_read or atomic_set. A common example where a memory
      barrier may be required is when atomic ops are used for reference