Add clarifying note for 3rd closure exercise

This pull request attempts to clarify the exercise instructions to avoid the need for call/apply.

When students attempt the memoize exercise, they can get stuck if they try to pass in an input function that has parameters because that require knowledge of `.call()` and `.apply()`. Do students already have knowledge of call and apply methods?

For example:
```JavaScript
var sum = function(a, b) {
  return a + b;
};

var modifiedSum = memoize(sum);
modifiedSum(); // will return NaN unless students know how to write memoize using .call() or .apply()
```