Fixed "typo" in example.

Fixing code "typo" in example.
In the original file, line 145 reads:
        url(r'^api-auth/', include('rest_framework.urls'),
It's missing the closing parenthesis.

Author: guntanis

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

@@ -142,7 +142,7 @@ Add the following import at the top of the file:
 And, at the end of the file, add a pattern to include the login and logout views for the browsable API.
 
     urlpatterns += [
-        url(r'^api-auth/', include('rest_framework.urls'),
+        url(r'^api-auth/', include('rest_framework.urls')),
     ]
 
 The `r'^api-auth/'` part of pattern can actually be whatever URL you want to use.