docs: luadns: Fix example keyword arguments

Change "key=" to "secret=" and "user=" to "key=".

Author: Aaron Sierra

docs/examples/dns/luadns/instantiate_driver.py

@@ -2,4 +2,4 @@
 from libcloud.dns.providers import get_driver
 
 cls = get_driver(Provider.LUADNS)
-driver = cls(user="user", key="api_key")
+driver = cls(key="user", secret="api_key")