Documentation/locking/locktypes: Fix PREEMPT_RT _bh() description

With PREEMPT_RT the _bh() version of a spinlock leaves preemption
enabled, align the doc to say that instead of the opposite.
Reported-by: Leah Leshchinsky <lleshchi@redhat.com>
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20220224212312.2601153-1-ahalaney@redhat.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/locking/locktypes.rst

@@ -247,7 +247,7 @@ based on rt_mutex which changes the semantics:
    Non-PREEMPT_RT kernels disable preemption to get this effect.
 
    PREEMPT_RT kernels use a per-CPU lock for serialization which keeps
-   preemption disabled. The lock disables softirq handlers and also
+   preemption enabled. The lock disables softirq handlers and also
    prevents reentrancy due to task preemption.
 
 PREEMPT_RT kernels preserve all other spinlock_t semantics: