perfect-numbers: set estimated difficulty to 3

config.json

       "topics": []
     },
     {
+      "slug": "perfect-numbers",
+      "difficulty": 3,
+      "topics": []
+    },
+    {
       "slug": "sum-of-multiples",
       "difficulty": 4,
       "topics": []
       "topics": []
     },
     {
-      "slug": "perfect-numbers",
-      "difficulty": 1,
-      "topics": []
-    },
-    {
       "slug": "change",
       "difficulty": 1,
       "topics": []