Update Letter Case Permutation from Easy to Medium difficulty (seanprashad#53)

Fixes seanprashad#52

Author: Sean Prashad

src/data/index.js

@@ -180,7 +180,7 @@ const questions = [
     name: 'Letter Case Permutation',
     url: 'https://leetcode.com/problems/letter-case-permutation/',
     pattern: ['Backtracking'],
-    difficulty: 'Easy',
+    difficulty: 'Medium',
     premium: false,
     companies: ['Amazon'],
   },