Fixed mistake in a variables.md code snippet (#238)

Fixed unintentional code mistake in a Javascript example in variables.md.

Co-authored-by: Jean-Philippe Sirois <[email protected]>

src/content/resources/javascript/variables.md

@@ -117,7 +117,7 @@ console.log(myVar); // 'test'
 This is fixed using `let`
 
 ```js
-for('let i = 0; i < 5; i++) {
+for(let i = 0; i < 5; i++) {
   console.log(i);
 }
 console.log(i); // Error: i is undefined