Update Mullvad swag link (#273)

Update Mullvad swag link to fix issue #272

- [x] I've checked that this isn't a new swag opportunity proposal.
- [x] I've checked that this isn't a duplicate pull request.

<!-- Describe your changes below -->
This will fix issue #272


<!-- Thanks for contributing! -->

data.json

@@ -156,7 +156,7 @@
     "name": "Mullvad",
     "difficulty": "easy",
     "description": "Send an email to Mullvad to get an assortment of stickers sent your way.",
-    "reference": "https://www.mullvad.net/en/guides/mullvad-stickers-and-merchandise/",
+    "reference": "https://mullvad.net/en/blog/2017/6/30/mullvad-merch-available/",
     "image": "https://www.mullvad.net/media/uploads/2017/10/20/mullvad-stickers.jpg",
     "dateAdded": "2018-03-11T16:47:14.000Z",
     "tags": ["stickers"]