PR changes: use const instead of let

typescript.html.markdown

@@ -229,18 +229,18 @@ numbers = moreNumbers; // Error, mutating methods are missing
 // for..of statement
 // iterate over the list of values on the object being iterated
 let arrayOfAnyType = [1, "string", false];
-for (let val of arrayOfAnyType) {
+for (const val of arrayOfAnyType) {
     console.log(val); // 1, "string", false
 }
 
 let list = [4, 5, 6];
-for (let i of list) {
+for (const i of list) {
    console.log(i); // "4", "5", "6"
 }
 
 // for..in statement
 // iterate over the list of keys on the object being iterated
-for (let i in list) {
+for (const i in list) {
    console.log(i); // "0", "1", "2",
 }