Use const for palindrome solution variable (#439)

09_palindromes/solution/palindromes-solution.js

@@ -1,6 +1,6 @@
 const palindromes = function (string) {
   // Since we only consider letters and numbers, create a variable containing all valid characters
-  let alphanumerical = 'abcdefghijklmnopqrstuvwxyz0123456789';
+  const alphanumerical = 'abcdefghijklmnopqrstuvwxyz0123456789';
 
   // Convert to lowercase, split to array of individual characters, filter only valid characters, then rejoin as new string
   const cleanedString = string