Clarified ambiguous instructions

[jared-wallace]

The instructions were somewhat ambiguous, for example my reading of it produced this function:

var add = function (a=0, b=0) {
    a + b;
};

(Great work on this btw)

[jared-wallace]

Second commit was because the original function would look like:

var totalCost = function(cost, quantity) {
    console.log(cost * quantity);
}