move to const to const

Author: shahata

src/2023/day21.spec.js

@@ -2,7 +2,7 @@ import { part1, part2 } from "./day21.js";
 import readInput from "../utils/read-input.js";
 
 let input = readInput(import.meta.url);
-let example = [
+const example = [
   "...........",
   ".....###.#.",
   ".###.##..#.",