Update 01_javascript_refresher.md

i to index

01_Day_JavaScript_Refresher/01_javascript_refresher.md

@@ -3409,7 +3409,7 @@ Let see different examples
 ```js
 const countries = ['Finland', 'Estonia', 'Sweden', 'Norway']
 countries.forEach(function (country, index, arr) {
-  console.log(i, country.toUpperCase())
+  console.log(index, country.toUpperCase())
 })
 ```