Updated Food-Chain Topics :) (#818)

* Updated Food-Chain Topics :)

* Removing plus signs from tags for food-chain

* minus comma

config.json

       "core": false,
       "difficulty": 6,
       "slug": "food-chain",
-      "topics": null,
+      "topics": [
+       	"arrays",
+       	"lists",
+       	"strings"
+       ],
       "unlocked_by": "two-fer",
       "uuid": "dd3e6fd6-5359-4978-acd7-c39374cead4d"
     },