doc: documentation/hwspinlock.txt fix typo

Fix a trivial typo in Documentation/hwspinlock.txt

Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>