Update config.json

Updated the difficulty of the slug

config.json

       ]
     },
     {
-      "slug": "rotational-cipher",
-      "difficulty": 1,
+      "slug": "rna-transcription",
+      "difficulty": 2,
       "topics": [
 
       ]
     },
     {
-      "slug": "rna-transcription",
+      "slug": "rotational-cipher",
       "difficulty": 2,
       "topics": [