docs/locking: Fix outdated section names

Commit 2e4f5382 ("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>