difference-of-squares: add topics (exercism#1108)

cmccandless · Nathan Parsons · commit 20b06368cd61 · 2017-11-30T10:44:45.000Z
Fixes exercism#1079
diff --git a/config.json b/config.json
@@ -169,7 +169,7 @@
       "unlocked_by": null,
       "difficulty": 1,
       "topics": [
-
+	"mathematics"
       ]
     },
     {
