Add "owner=1" to final http example

The http POST command on line 209 currently fails with because it lacks the required key owner.
```
$ http -a admin:password POST http://127.0.0.1:8000/snippets/ code="print 789"
HTTP/1.0 400 BAD REQUEST
Allow: GET, POST, HEAD, OPTIONS
Content-Type: application/json
Date: Sat, 14 Mar 2015 16:37:05 GMT
Server: WSGIServer/0.2 CPython/3.4.2
Vary: Accept, Cookie
X-Frame-Options: SAMEORIGIN

{
    "owner": [
        "This field is required."
    ]
}
```

docs/tutorial/4-authentication-and-permissions.md

@@ -206,7 +206,7 @@ If we try to create a snippet without authenticating, we'll get an error:
 
 We can make a successful request by including the username and password of one of the users we created earlier.
 
-    http -a tom:password POST http://127.0.0.1:8000/snippets/ code="print 789"
+    http -a tom:password POST http://127.0.0.1:8000/snippets/ code="print 789" owner=1
 
     {
         "id": 5,