Fixed C#'s function tutorial's solution

The C# function solution doesn't work out of the box. I fixed the solution to work out of the box and also added a hint for the student to know to use public static modifiers. Obviously there are many other ways to solve this problem so it works, but, given what is expected to be known at this point in the tutorial, just telling them what to put seems better.

Though please do help me with the way I worded the tip. Or maybe consider adding a 'tip' style or whatever.

Thanks.