Updated Freelancer Rates exercise instructions.

[angiokas]

Added information about using the library to be able to use floor() and ceil() functions for part 3 and 4 of this exercise.

[github-actions]

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[vaeng]

Hey there,

thanks for the PR. I think you are right, we should improve the exercise. It is already in the hints for the exercise, but maybe we should be more explicit. I am hesitant to put it as directly as you have proposed though.