fix rwlock usage example

This is a trivial patch which fixes a typo on rwlock usage under
Documentation/spinlocks.txt.

Signed-Off-By: Lucas Correia Villa Real <lucasvr@gobolinux.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
diff --git a/Documentation/spinlocks.txt b/Documentation/spinlocks.txt
index c212299..a661d68 100644
--- a/Documentation/spinlocks.txt
+++ b/Documentation/spinlocks.txt
@@ -9,7 +9,7 @@
    static int __init xxx_init(void)
    {
    	spin_lock_init(&xxx_lock);
-	rw_lock_init(&xxx_rw_lock);
+	rwlock_init(&xxx_rw_lock);
 	...
    }