protein-translation: update unlocked by

It was previously unlocked by rna-transcription which is not a core exercise.
Exercises must be unlocked by core exercises.

config.json

         "loops",
         "strings"
       ],
-      "unlocked_by": "rna-transcription",
+      "unlocked_by": "two-fer",
       "uuid": "331073b3-bd1a-4868-b767-a64ce9fd9d97"
     },
     {