fix api_view decorator useage

docs/api-guide/authentication.md

@@ -50,7 +50,7 @@ You can also set the authentication policy on a per-view basis, using the `APIVi
 
 Or, if you're using the `@api_view` decorator with function based views.
 
-    @api_view(('GET',)),
+    @api_view(['GET'])
     @authentication_classes((SessionAuthentication, UserBasicAuthentication))
     @permissions_classes((IsAuthenticated,))
     def example_view(request, format=None):