fix rwlock usage example

This is a trivial patch which fixes a typo on rwlock usage under
Documentation/spinlocks.txt.
Signed-Off-By: NLucas Correia Villa Real <lucasvr@gobolinux.org>
Signed-off-by: NAdrian Bunk <bunk@stusta.de>

Documentation/spinlocks.txt

@@ -9,7 +9,7 @@ removed soon. So for any new code dynamic initialization should be used:
    static int __init xxx_init(void)
    {
    	spin_lock_init(&xxx_lock);
-	rw_lock_init(&xxx_rw_lock);
+	rwlock_init(&xxx_rw_lock);
 	...
    }