Fix typo in push notification devices viewset

Gijs Hendriksen
2018-07-11T21:24:15+02:00

#### Previous behaviour
Website crashes when updating push notifications categories.

Steps to reproduce:

1. Send a POST request containing `receive_category` to `https://thalia.nu/api/v1/devices/`.
2. Receive 500 error.

#### New behaviour
Website saves new preferences for push notifications categories.

Steps to validate that it works:

1. Send a POST request containing `receive_category` to `https://thalia.nu/api/v1/devices/`.
2. Don't receive 500 error.