docs/locking: Fix outdated section names

Commit 2e4f538 ("locking/doc: Rename LOCK/UNLOCK to
ACQUIRE/RELEASE") has not appied to 'spinlock.rst'.  This commit updates
the doc for the change.

Signed-off-by: SeongJae Park <sjpark@amazon.com>
Link: https://lore.kernel.org/r/20200131205237.29535-2-sj38.park@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/locking/spinlocks.rst

@@ -25,9 +25,9 @@ worry about UP vs SMP issues: the spinlocks work correctly under both.
 
      Documentation/memory-barriers.txt
 
-       (5) LOCK operations.
+       (5) ACQUIRE operations.
 
-       (6) UNLOCK operations.
+       (6) RELEASE operations.
 
 The above is usually pretty simple (you usually need and want only one
 spinlock for most things - using more than one spinlock can make things a