added topics for difference-of-squares

config.json

       "core": true,
       "difficulty": 3,
       "slug": "difference-of-squares",
-      "topics": null,
+      "topics": [
+        "loops",
+        "integers",
+        "mathematics"
+      ],
       "unlocked_by": null,
       "uuid": "b0da59c6-1b55-405c-b163-007ebf09f5e8"
     },