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