fix(learn): Added hint for 'this' keyword (#54653)

Co-authored-by: Huyen Nguyen <[email protected]>

curriculum/challenges/english/15-javascript-algorithms-and-data-structures-22/learn-basic-oop-by-building-a-shopping-cart/63f03686c5ea863533ec71f4.md

@@ -18,7 +18,7 @@ const afterCalculateTotal = code.split('calculateTotal')[1];
 assert.match(afterCalculateTotal, /const\s+tax\s*=/);
 ```
 
-Assign the value of calling your new `.calculateTaxes()` method, passing `subTotal` as the argument, to the `tax` variable.
+Assign the value of calling your new `.calculateTaxes()` method, passing `subTotal` as the argument, to the `tax` variable. Remember to use the `this` keyword.
 
 ```js
 const afterCalculateTotal = code.split('calculateTotal')[1];