Replace 'x' variable with 't' for Math function section

[mehlian]

At the beginning we can see graph that shows change in x based on t

But in 'Math function' section 'x' represents value that we should pass. It is confusing. Letter 't' should be used as parameter instead.
We pass 't' and receive 'x' after all.

[Michaelangel007]

In my Code Poetry: Easing Tutorial & Optimizations, section Parameter t or p, I use the variable p since it is a normalized percent to minimize confusion with t = time which isn't normalized.