Fix NULL builder config documentation.

Change from "host" to "ssh_host" was introduced in d545431f

website/source/docs/builders/null.html.markdown

@@ -21,7 +21,7 @@ no provisioners are defined, but it will connect to the specified host via ssh.
 ```javascript
 {
   "type":         "null",
-  "host":     "127.0.0.1",
+  "ssh_host":     "127.0.0.1",
   "ssh_username": "foo",
   "ssh_password": "bar"
 }