config.json in trackler-2.2.1.72

- old
+ new

@@ -167,10 +167,10 @@
       "slug": "difference-of-squares",
       "core": false,
       "unlocked_by": null,
       "difficulty": 1,
       "topics": [
-
+	"mathematics"
       ]
     },
     {
       "uuid": "43eaf8bd-0b4d-4ea9-850a-773f013325ef",
       "slug": "anagram",