added topics to perfect numbers (#863)

* added topics to perfect numbers

* updated topics

* fixed tabs

config.json

@@ -156,7 +156,13 @@
       "core": false,
       "difficulty": 3,
       "slug": "perfect-numbers",
-      "topics": null,
+      "topics": [
+        "integers",
+        "mathematics",
+        "exception_handling",
+        "enumerations",
+        "filtering"
+      ],
       "unlocked_by": "difference-of-squares",
       "uuid": "d0dcc898-ec38-4822-9e3c-1a1829c9b2d9"
     },