Fix NS1 provider name

Update example class name from "octodns.provider.nsone.Ns1Provider" to working "octodns.provider.ns1.Ns1Provider".

octodns/provider/ns1.py

@@ -18,7 +18,7 @@ class Ns1Provider(BaseProvider):
     Ns1 provider
 
     nsone:
-        class: octodns.provider.nsone.Ns1Provider
+        class: octodns.provider.ns1.Ns1Provider
         api_key: env/NS1_API_KEY
     '''
     SUPPORTS_GEO = False