list-ops: estimate difficulty 9

config.json

       ]
     },
     {
-      "slug": "custom-set",
-      "difficulty": 10,
+      "slug": "list-ops",
+      "difficulty": 9,
       "topics": [
-
+      
       ]
     },
     {
-      "slug": "list-ops",
-      "difficulty": 1,
+      "slug": "custom-set",
+      "difficulty": 10,
       "topics": [
-      
+
       ]
     }
   ],