Update math.md, simplifying secondsToRadiant return operation

[tuco5]

I want to propose this change to simplify the lecture of the math chapter.

By simplifying the radiant division in the secondsToRadiant and use the parameter as numerator instead of denominator we avoid handling a 0 case type.

Benefits:

• We can short the chapter removing the zero division error explanation.
• By dividing the func parameter Is easier to read and we avoid the imprecise floating points
• If func param is 0 then the return value will be 0 instead of an error.

I hope this helps to simplify the code and the lecture.
Thanks for this great course