Documentation/locking/locktypes: Fix a typo

We have three categories locks, not two.
Signed-off-by: Huang Shijie <sjhuang@iluvatar.ai>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200813060220.18199-1-sjhuang@iluvatar.ai

Documentation/locking/locktypes.rst

@@ -10,7 +10,7 @@ Introduction
 ============
 
 The kernel provides a variety of locking primitives which can be divided
-into two categories:
+into three categories:
 
  - Sleeping locks
  - CPU local locks