two-fer: add topics

config.json

       "core": true,
       "difficulty": 1,
       "slug": "two-fer",
-      "topics": null,
+      "topics": [
+        "strings"
+      ],
       "unlocked_by": null,
       "uuid": "74515d45-565b-4be2-96c4-77e58efa9257"
     },